(Bug 48941) Normalize whitespace in ext-tag attribute values.

* PHP parser normalizes whitespace in attribute values of all
  extension tags.  This patch mimics that behavior.

* Removed special case whitespace stripping for references.group
  attribute since that is now handled by the general case.

* As a result, <ref name='foo' /> and <ref name=' foo  ' /> are
  and treated as being identical.

Change-Id: I34009ab6662d05453fe46379c58d6e989f296958
Merge "(Bug 48941) Normalize whitespace in ext-tag attribute values."