REST: Check.user is not read-only

REST: Check.user is not read-only

We only support 'Check' creation - not check updating. As a result,
there's no real reason that the 'Check.user' field should be read-only
and this is causing an issue with Django REST Framework 3.7. Simply
remove the attribute and extend the tests to validate things are working
as expected.

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