Further test coverage

* Fixes `LocationHeaderMixin`
* Fixes `MethodNotAllowed`
* coverage for `antfarm.response` is now at 92.1%
* coverage for `antfarm.request` is now at 93.5%
* overall coverage for doing `coverage run tests/__main__.py` on my machine now reports 93.9%

Obviously the coverage doesn't mean everything is actually under test, but getting to 100% is a good goal to start with ;)

Remaining untested:
* `antfarm.response.Response.build_headers`
* The `Morsel` branch of `antfarm.response.Response.add_cookie` - I think it might be wrong, as `Morsel.set` only takes `coded_value` as it's third argument, even though it feels like it should star expand them into the rest of the attrs it needs.
* `antfarm.response.Response.__iter__` still has only partial branch coverage
* `antfarm.request.Request.request_data` - as discussed, I'm not sure `parse_multipart` is receiving the right thing -- it should possibly be taking `self.content_params` wholesale.

(Edit: rebased against master)