Reinsert leading space properly when stripping indent-pres.

* The handlePres DOM post-processor pass strips indent-pres from
  block nodes, but it was not correctly adding back a leading
  space on all new lines -- it was only adding one leading space
  before the first child.

* Fixed the code to process all text nodes nested under the
  indent-pre and add back the leading space on all new lines.

* Correct RTs the following contrived wikitext
-----------
<blockquote>
a
 b
<span>c</span>
 <span>c
 d
 e
 </span>
 <span>f
 g
 h
</span>
 <p>
  i
 </p>
j
</blockquote>

<pre>
 a
</pre>
-----------

* No change in parser tests (TODO: add above or a variant)

* Correctly RTs en:Idents

Change-Id: I42db765a54518f4457b1a263671743c1438398c1
Merge "Reinsert leading space properly when stripping indent-pres."