Fix source checking when parsing {{~!~}} and the like.

[navels]

This fixes a warning that derives from several of these statements in ember-prism, which are intended to strip whitespace without otherwise doing anything:

{{~! ~}}

Presently these result in a compiler warning when building ember-prism or any addon which consumes it:

unexpectedly found "! ~" when slicing source, but expected " "
unexpectedly found "! ~" when slicing source, but expected " "
unexpectedly found "! ~" when slicing source, but expected " "
unexpectedly found "! ~" when slicing source, but expected " "

Fixes one cause of https://github.com/emberjs/ember.js/issues/19392

When fixing this I decided some tests would be nice to add even though they don't cover this fix. Not really thinking it is worth a test specifically targeting this case, though.