fix(ngSanitize): ensure &nbsp; is converted to non-breaking space in IE8

IE8 does not convert &nbsp; to a non-breaking space when set using innerHTML,
and instead converts it to a single U+0020 space character, which causes
whitespace to collapse in most HTML elements.

This CL explicitly converts &nbsp; with U+00A0 in order to prevent this.

Closes #8602
IE8 doesn't think non-breaking space counts as a space character in regexps, for some reason. But this bit might not be totally necessary, since we explicitly replace `&nbsp;` with `\u00a0` further down.