[cssom] Have property setters append new declaration rather than set in place.

This resolves #1898.

Author: upsuper

cssom-1/Overview.bs

@@ -2209,8 +2209,8 @@ a list of declarations <var>declarations</var>, follow these steps:
 
 <ol>
  <li>If <var>property</var> is a <a>case-sensitive</a> match for a <a for="CSS declaration">property
- name</a> of a <a>CSS declaration</a> in <var>declarations</var>, let <var>declaration</var> be that <a>CSS declaration</a>.
- <li>Otherwise, append a new <a>CSS declaration</a> with the <a for="CSS declaration">property name</a> <var>property</var>
+ name</a> of a <a>CSS declaration</a> in <var>declarations</var>, remove that <a>CSS declaration</a>.
+ <li>Append a new <a>CSS declaration</a> with the <a for="CSS declaration">property name</a> <var>property</var>
  to <var>declarations</var> and let <var>declaration</var> be that <a>CSS declaration</a>.
  <li>Set <var>declaration</var>'s <a for="CSS declaration">value</a> to <var>component value list</var>.
  <li>If the <i>important</i> flag is set, set <var>declaration</var>'s <a for="CSS declaration">important flag</a>.