langs/i18n: Revise the plural implementation

There were some issues introduced with the plural counting when we upgraded from v1 to v2 of go-i18n.

This commit improves that situation given the following rules:

* A single integer argument is used as plural count and passed to the i18n template as a int type with a `.Count` method. The latter is to preserve compability with v1.
* Else the plural count is either fetched from the `Count`/`count` field/method/map or from the value itself.
* Any data type is accepted, if it can be converted to an integer, that value is used.

The above means that you can now do pass a single integer and both of the below will work:

```
{{ . }} minutes to read
{{ .Count }} minutes to read
```

Fixes #8454
Closes #7822
See https://github.com/gohugoio/hugoDocs/issues/1410

langs/i18n/i18n_test.go

@@ -141,6 +141,20 @@ other = "{{ .Count }} minutes to read"
 		expected:     "One minute to read",
 		expectedFlag: "One minute to read",
 	},
+	{
+		name: "readingTime-many-dot",
+		data: map[string][]byte{
+			"en.toml": []byte(`[readingTime]
+one = "One minute to read"
+other = "{{ . }} minutes to read"
+`),
+		},
+		args:         21,
+		lang:         "en",
+		id:           "readingTime",
+		expected:     "21 minutes to read",
+		expectedFlag: "21 minutes to read",
+	},
 	{
 		name: "readingTime-many",
 		data: map[string][]byte{
@@ -155,6 +169,62 @@ other = "{{ .Count }} minutes to read"
 		expected:     "21 minutes to read",
 		expectedFlag: "21 minutes to read",
 	},
+	// Issue #8454
+	{
+		name: "readingTime-map-one",
+		data: map[string][]byte{
+			"en.toml": []byte(`[readingTime]
+one = "One minute to read"
+other = "{{ .Count }} minutes to read"
+`),
+		},
+		args:         map[string]interface{}{"Count": 1},
+		lang:         "en",
+		id:           "readingTime",
+		expected:     "One minute to read",
+		expectedFlag: "One minute to read",
+	},
+	{
+		name: "readingTime-string-one",
+		data: map[string][]byte{
+			"en.toml": []byte(`[readingTime]
+one = "One minute to read"
+other = "{{ . }} minutes to read"
+`),
+		},
+		args:         "1",
+		lang:         "en",
+		id:           "readingTime",
+		expected:     "One minute to read",
+		expectedFlag: "One minute to read",
+	},
+	{
+		name: "readingTime-map-many",
+		data: map[string][]byte{
+			"en.toml": []byte(`[readingTime]
+one = "One minute to read"
+other = "{{ .Count }} minutes to read"
+`),
+		},
+		args:         map[string]interface{}{"Count": 21},
+		lang:         "en",
+		id:           "readingTime",
+		expected:     "21 minutes to read",
+		expectedFlag: "21 minutes to read",
+	},
+	{
+		name: "argument-float",
+		data: map[string][]byte{
+			"en.toml": []byte(`[float]
+other = "Number is {{ . }}"
+`),
+		},
+		args:         22.5,
+		lang:         "en",
+		id:           "float",
+		expected:     "Number is 22.5",
+		expectedFlag: "Number is 22.5",
+	},
 	// Same id and translation in current language
 	// https://github.com/gohugoio/hugo/issues/2607
 	{
@@ -246,6 +316,46 @@ func doTestI18nTranslate(t testing.TB, test i18nTest, cfg config.Provider) strin
 	return f(test.id, test.args)
 }
 
+type countField struct {
+	Count int
+}
+
+type noCountField struct {
+	Counts int
+}
+
+type countMethod struct {
+}
+
+func (c countMethod) Count() int {
+	return 32
+}
+
+func TestGetPluralCount(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(getPluralCount(map[string]interface{}{"Count": 32}), qt.Equals, 32)
+	c.Assert(getPluralCount(map[string]interface{}{"Count": 1}), qt.Equals, 1)
+	c.Assert(getPluralCount(map[string]interface{}{"Count": "32"}), qt.Equals, 32)
+	c.Assert(getPluralCount(map[string]interface{}{"count": 32}), qt.Equals, 32)
+	c.Assert(getPluralCount(map[string]interface{}{"Count": "32"}), qt.Equals, 32)
+	c.Assert(getPluralCount(map[string]interface{}{"Counts": 32}), qt.Equals, 0)
+	c.Assert(getPluralCount("foo"), qt.Equals, 0)
+	c.Assert(getPluralCount(countField{Count: 22}), qt.Equals, 22)
+	c.Assert(getPluralCount(&countField{Count: 22}), qt.Equals, 22)
+	c.Assert(getPluralCount(noCountField{Counts: 23}), qt.Equals, 0)
+	c.Assert(getPluralCount(countMethod{}), qt.Equals, 32)
+	c.Assert(getPluralCount(&countMethod{}), qt.Equals, 32)
+
+	c.Assert(getPluralCount(1234), qt.Equals, 1234)
+	c.Assert(getPluralCount(1234.4), qt.Equals, 1234)
+	c.Assert(getPluralCount(1234.6), qt.Equals, 1234)
+	c.Assert(getPluralCount(0.6), qt.Equals, 0)
+	c.Assert(getPluralCount(1.0), qt.Equals, 1)
+	c.Assert(getPluralCount("1234"), qt.Equals, 1234)
+	c.Assert(getPluralCount(nil), qt.Equals, 0)
+}
+
 func prepareTranslationProvider(t testing.TB, test i18nTest, cfg config.Provider) *TranslationProvider {
 	c := qt.New(t)
 	fs := hugofs.NewMem(cfg)