Bug/crash/feature fixes for submitting forms.

Now longer crashes when attempting to save cookies either in-memory or on-disk.
On-disk cookie storage is now resilient to changes in http-client internals.
HURL now handles encoding form submissions as per caller-specified MIMEtype.
HURL now supports a choice of HTTP method when submitting forms.

Bugfix means now content negotiation, caching, & form data are now actually sent to the server.