[cssom] Serialize an identifier should handle '-' and '--'. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25426

Author: zcorpan

cssom/Overview.bs

@@ -198,6 +198,13 @@ by the concatenation of, for each character of the identifier:
  (U+0030 to U+0039) and the first character is a "<code>-</code>"
  (U+002D), then the character
  <a>escaped as code point</a>.
+ <li>If the character is the first character and is a "<code>-</code>" (U+002D),
+ and there is no second character,
+ then the <a lt="escape a character">escaped</a> character.
+ <li>If the character is the second character and is a "<code>-</code>" (U+002D),
+ and the first character is also a "<code>-</code>" (U+002D),
+ and there is no third character,
+ then the <a lt="escape a character">escaped</a> character.
  <li>If the character is not handled by one of the above rules and is
  greater than or equal to U+0080, is "<code>-</code>" (U+002D) or
  "<code>_</code>" (U+005F), or is in one of the ranges \[0-9] (U+0030 to