make_clickable() should not operate on links in attributes.

[ryan]

make_clickable() turns this:

<a href="http://example.com/example.gif" title="Image from http://example.com">Look at this image!</a>

into this:

<a href="http://example.com/example.gif" title="Image from <a href="http://example.com" rel="nofollow">http://example.com</a>">Look at this image!</a>

It should not.

In [20443]:

Don't attempt to make links inside attributes clickable. Props duck_ azaozz. fixes #20418

​http://unit-tests.trac.wordpress.org/changeset/667

Looks good, tests well.