Fixes deleteWordForward at line ending (#2290)

#### Is this adding or improving a _feature_ or fixing a _bug_?
Bug fix - Very minor change

#### What's the new behavior?

`deleteWordBackward` at line ending behaves the same as `deleteCharForward`

#### How does this change work?

If `TextUtils.getWordOffsetForward(text, o)` returns 0, instead we substitute in `1` to delete the line break and join the blocks. The relevant test for this change (`join-blocks`) is also added.

#### Have you checked that...?

<!-- 
Please run through this checklist for your pull request: 
-->

* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)

#### Does this fix any issues or need any specific reviewers?

Fixes: #2258
Reviewers: @ianstormtaylor

packages/slate/src/commands/at-range.js

@@ -455,7 +455,8 @@ Commands.deleteWordForwardAtRange = (change, range) => {
   const offset = startBlock.getOffset(start.key)
   const o = offset + start.offset
   const { text } = startBlock
-  const n = TextUtils.getWordOffsetForward(text, o)
+  const wordOffset = TextUtils.getWordOffsetForward(text, o)
+  const n = wordOffset === 0 ? 1 : wordOffset
   change.deleteForwardAtRange(range, n)
 }
 

packages/slate/test/commands/at-current-range/delete-word-forward/join-blocks.js

@@ -0,0 +1,28 @@
+/** @jsx h */
+
+import h from '../../../helpers/h'
+
+export default function(change) {
+  change.deleteWordForward()
+}
+
+export const input = (
+  <value>
+    <document>
+      <paragraph>
+        word<cursor />
+      </paragraph>
+      <paragraph>another</paragraph>
+    </document>
+  </value>
+)
+
+export const output = (
+  <value>
+    <document>
+      <paragraph>
+        word<cursor />another
+      </paragraph>
+    </document>
+  </value>
+)