Cannot insert text at beginning of text input when there is text

[aaronmensemble]

Lightning-ui/src/Keyboard.js

Line 203 in 011c938

event.index = this._inputField.cursorIndex;

This line of code does not seem to be hit when index is set to 0 when there is existing text.

Steps:

1. In an input, enter some text: "test|" (where '|' is cursor)
2. Move cursor left 2 times: "te|st"
3. Enter text 'a': "tea|st" - Note here that text enters correctly because the condition on line 202 sees cursorIndex as valid (ie 2)

However, if you move the cursor all the way to beginning: "|test"
The condition now fails because cursorIndex = 0 is falsey. So line 203 is skipped and defaults to this._input.length, which will add to end.