Only generate an attribute prefix if required

If implemented as specified, the prefix set for an attribute in the DOM
would never be preserved during serialization. This change only
generates the new prefix if the prefix is either missing (which is not
allowed for namespaced attributes), or if it conflicts with another
local namespace definition of the prefix for a different namespace.

Fixes #29

index.html

@@ -1358,19 +1358,21 @@ <h2>Extensibility</h2>
           "<code>xmlns</code>".
         </ol>
 
-        <li>Otherwise, the <var>attribute namespace</var> in not the <a>XMLNS namespace</a>. Run
-        these steps:
+        <li>Otherwise, the <var>attribute namespace</var> is not the <a>XMLNS namespace</a>. If
+        <var>candidate prefix</var> is null, run these steps:
         <ol>
-          <li>Let <var>candidate prefix</var> be the result of <a>generating a prefix</a> providing
-          <var>map</var>, <var>attribute namespace</var>, and <var>prefix index</var> as input.
+         <li>Let <var>new prefix</var> be <var>attr</var>'s <code><a>prefix</a></code> attribute if it is
+         not <code>null</code> and the <var>local prefixes map</var> does not contain a key matching
+         <var>attr</var>'s <code><a>prefix</a></code>, or the result of <a>generating a prefix</a> providing
+         <var>map</var>, <var>attribute namespace</var>, and <var>prefix index</var> as input otherwise.
 
           <li>Append the following to <var>result</var>, in the order listed:
           <ol>
             <li>"<code> </code>" (U+0020 SPACE);
 
             <li>The string "<code>xmlns:</code>";
 
-            <li>The value of <var>candidate prefix</var>;
+            <li>The value of <var>new prefix</var>;
 
             <li>"<code>="</code>" (U+003D EQUALS SIGN, U+0022 QUOTATION MARK);