Allow deleteAtRange with a zero-length range on the first character (#2800)

deleteAtRange will consider a zero-length range on the first character
of a text node as a hanging selection, which is incorrect. This should
not be considered hanging.

It's still possible to hit the `startKey === endKey && isHanging`
conditional if endKey is in a void node, since we will bump the
selection to the previous node and update endKey (but then endOffset
is no longer 0, so it's not _really_ hanging anymore). We have an
existing test for that, and it still passes after this change.

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

@@ -114,7 +114,8 @@ Commands.deleteAtRange = (editor, range) => {
     endOffset === 0 &&
     isStartVoid === false &&
     startKey === startBlock.getFirstText().key &&
-    endKey === endBlock.getFirstText().key
+    endKey === endBlock.getFirstText().key &&
+    startKey !== endKey
 
   // If it's a hanging selection, nudge it back to end in the previous text.
   if (isHanging && isEndVoid) {

packages/slate/test/commands/at-current-range/delete/first-position.js

@@ -0,0 +1,29 @@
+/** @jsx h */
+
+import h from '../../../helpers/h'
+
+export default function(editor) {
+  editor.delete()
+}
+
+export const input = (
+  <value>
+    <document>
+      <paragraph>
+        <cursor />
+        word
+      </paragraph>
+    </document>
+  </value>
+)
+
+export const output = (
+  <value>
+    <document>
+      <paragraph>
+        <cursor />
+        word
+      </paragraph>
+    </document>
+  </value>
+)