Fix #1723: Parsing of a compound selector made of only a pseudo-element
auto merge of #1724 : SimonSapin/servo/pseudo-element-parsing, r=larsbergstrom
Prevent '&nbsp' from stripping as whitespace
Fix: whitespace is considered as spaces(U+0020), tabs(U+0009), and line breaks(U+000D U+000A)
auto merge of #1728 : june0cho/servo/whitespace_nbsp, r=SimonSapin

Fix #1727.
Merge remote-tracking branch 'upstream/master'