Clean up http classes a bit

Clean up http classes a bit

* added integration tests. We probably don't want automated tests
  to make external requests but these make manual testing more
  convenient. Documented some oddities discovered by testing.
* made ::$status, ::proxySetup() and ::getHeaderList()
  protected; they were not referenced in any gerrit-hosted extension
  and they provide no useful functionality to external callers.
  Similarly, marked ::read() and ::errorHandler() as internal
  (these are used as callbacks so can't be protected)
* removed inheritance abuse in ::execute()
* documented ::execute() as returning a StatusValue (but
  keep returning a Status for now)
* changed setCookie argument defaults to ones that make sense
* replaced MWException
* moved unit tests to the correct location
* fixed some code style issues

Change-Id: I5852fc75badc5d475ae30ec2c9376bde7024bd95