Refs #369: Fix header handling during cache revalidation.

See:

 * [rfc2616 13.5.1](http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.5.1)
 * [httplib2.py](https://github.com/jcgregorio/httplib2/blob/df1cca48b204fc20fde1e49d7ad763394b01d931/python2/httplib2/__init__.py#L197-L203)