Add a falling test case for list of objectid

Hello Nicola,
I added a falling test for a list of objectid.

The main idea is to have a tag collection. The user then can have a list of tags.

I did not found test cases for list with referential integrity, so I added one.

Please have a look. Maybe I am doing something wrong.

```
nosetests eve/tests/methods/patch.py:TestPatch.test_patch_list_referential_integrity
F
======================================================================
FAIL: test_patch_list_referential_integrity (eve.tests.methods.patch.TestPatch)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/daniele/ahref/dev/eve/eve/tests/methods/patch.py", line 253, in test_patch_list_referential_integrity
    msg="key1: %s" % r['key1'])
AssertionError: key1: {'status': 'ERR', 'issues': ["'tags': length of list should be 1"]}

----------------------------------------------------------------------
Ran 1 test in 0.463s

FAILED (failures=1)
```