Too much whitespace was getting deleted

In several places the code was using Apple's whitespaceAndNewlineCharacterSet for ignorable
whitespace, but it should have been using the internal ignorableWhitespaceCharacterSet.

ignorableWhitespaceCharacterSet was also trying to modify an immutable object, which clearly wasn't working.

These bugs were causing HTML like:

&amp;nbsp;&lt;br>

to be ignored completely in many (but not all) situations.