Fixing put(), and strenghtening unittests.

* Removing unused inports in test_field
* XXX. have to ask about the standard json 'ok' response in globaleaks
* Fix fieldscollection.put handler.

Note: models.Field.add_children is never used in handlers. It should be slightly
modified to be used in the put transaction
Note: I am not so sure we really assert that the a child of type≠fieldgroup is
not allowed to have children, but hey, tests are working.