REST: Allow for mutability of request.POST

REST: Allow for mutability of request.POST

Using Django 1.11 yields the following error for the 'patchwork.tests
.test_rest_api.TestCheckAPI.test_create' test:

    AttributeError: This QueryDict instance is immutable

This occurs due to our modification of data to allow users to create
instances using a slugified state instead of the underlying integer
value, e.g. 'success' instead of 1.

Resolve this by unsetting the immutability of that queryset. As
suggested in the linked SO answer, there is limited side effects to
doing this.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Tested-by: Daniel Axtens <dja@axtens.net>