Fix some issues that popped up when running against HPCloud.

* Check that text/html is not actually xml.
* Strip leading/trailing whitespaced from xml text.
* Check for 'retry-after' in HTTP413 response header in
  addition to 'retryAfter' in HTTP413 response body.

Change-Id: Iddd7e40388a2a39e986fb519d3cbbf1ff9874c6f