Serialize image-spans without hacking the span tag.

* The serializer code had a long-standing hack where the span node
  was modified by setting rel and href attributes on it so as to
  reuse the link-handler set on the a-tag.

* This hack then introduced artificial semantic errors in roundtrip
  tests on the href='' attribute set on the span (modification of DOM
  input to the serializer).  Try the following roundtrip test.

node roundtrip-test.js --prefix pl Mistrzostwa_Słowacji_w_Lekkoatletyce_2011

* This patch eliminates the hack since it is no longer necessary
  and the span tag handler can handle mw:Image spans correctly.

* No change in parser test results, but eliminates false semantic
  error reports on the above page (and possibly other pages as well).

Change-Id: Icb94f8298328685c0f006a975308da47df2a0ebd
Merge "Serialize image-spans without hacking the span tag."