default "space" when not overwriting so autocorrect works on Mobile Safari

If no range is selected when the user hits "space" -- and hence is not "overwriting" a selection -- then we should allow the "space" to pass through as default rather than inserting '\u00A0'. Allowing the space to be treated default makes auto-correct continue to work on (at least) iPhones, where the "space" event is what triggers auto-correct, auto-complete, auto-capitalization, 2 spaces marking the end of a sentence, etc. Otherwise, auto-correct breaks.