TEXT_CHANGE event generates incorrect Delta when inserting same character

[sachinrekhi]

When you insert a new character right before the same existing character, Quill incorrectly creates a Delta suggesting that the existing character was added, not the new character.

While this technically results in the same document, it does result in certain issues. For example, when I'm trying to re-implement MultiCursor support, it's resulting in shiftCursors() not moving the cursor to the new correct position since the generated delta incorrectly indicates which character was added. This will also ultimately create issues for the authorship module as well on exactly who inserted what character.

Steps for Reproduction

1. Visit beta.quilljs.com
2. Console: quill.setContents([{insert:'ttest\n'}])
3. Console: quill.on(Quill.events.TEXT_CHANGE, (delta) => {console.dir(delta)})
4. Enter "t" and the beginning of the contents of the editor

Expected behavior:
Console shows that the generated Delta is simply: [{insert: "t"}]

Actual behavior:
Actual generated delta is: [{retain: 2}, {insert: "t"}]

Platforms:
Chrome 51 on Mac 10.11

Version:
Quill 1.0 Beta 4