Added tests for Django 1.6 QuerySet.count() capability

Tests for issue https://github.com/Suor/django-cacheops/issues/54

Its passes on Django<1.6
and fails on Django=1.6 with

```
ERROR: test_54 (tests.tests.IssueTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/hell/projects/django/django-cacheops/tests/tests.py", line 127, in test_54
    issues.count()
  File "/home/hell/projects/django/django-cacheops/cacheops/query.py", line 369, in count
    if self._result_cache is not None and not self._iter:
AttributeError: 'QuerySet' object has no attribute '_iter
```