Smart paste when pasting a word with trailing whitespace after a word and a space before another word.

To run this test manually, select "hello " (including trailing space), copy, and paste.

Expected result

A space should be added between the preexisting word and the word that's pasted. No space should be added after the pasted word: hello hello world

Actual result

execCopyCommand: <div id="test" class="editing" style="font-size: 24px;"> hello world </div>
execPasteCommand: <div id="test" class="editing" style="font-size: 24px;"> hello&nbsp;hello&nbsp;world</div>
