URLs within an inline HTML <a> element should not be processed by AUTOLINKS

[djneades]

In pegdown 1.2.1 with Extensions.AUTOLINKS enabled, the following Markdown containing an inline HTML <a> element:

This <a rel="external" href="http://www.example.com/">http://www.example.com/</a> line contains an inline HTML element.

renders to this malformed HTML:

<p>This <a rel="external" href="http://www.example.com/"><a href="http://www.example.com/</a">http://www.example.com/</a</a> line contains an inline HTML element.</p>

AUTOLINKS ought not to process a URL if it is already within an inline HTML <a> element.