Fixed issue where get_all_keys or _get_occurence choked on strings.

Created by: jpavlav

Found a bug when trying to utilize get_occurence_of_key and get_all_keys on a data structure similar to this:

     self.sample4 = {
        "values": [
            {
                "checks": [
                    {
                        "monitoring_zones": [
                            "mzdfw",
                            "mzfra",
                            "mzhkg",
                            "mziad",
                            "mzlon",
                            "mzord",
                            "mzsyd"
                        ]
                    }
                ]
            }
        ]
    }

I've added a test to the test_nested_lookup file that shows the error in question, but for ease, this is what is returned when the test is run without the fix:

python -m unittest test_nested_loopkup ...E...........

ERROR: test_sample_data4 (test_nested_loopkup.TestGetAllKeys)

Traceback (most recent call last): File "test_nested_loopkup.py", line 245, in test_sample_data4 File "nested_lookup/nested_lookup.py", line 63, in get_all_keys recrusion(dictionary=dictionary) File "nested_lookup/nested_lookup.py", line 59, in recrusion recrusion(dictionary=list_items) File "nested_lookup/nested_lookup.py", line 59, in recrusion recrusion(dictionary=list_items) File "nested_lookup/nested_lookup.py", line 59, in recrusion recrusion(dictionary=list_items) File "nested_lookup/nested_lookup.py", line 52, in recrusion for key, value in iteritems(dictionary): File "/Users/jpavlav/Projects/nested-lookup/venv2/lib/python2.7/site-packages/six.py", line 605, in iteritems return d.iteritems(**kw) AttributeError: 'str' object has no attribute 'iteritems'

---

Ran 15 tests in 0.001s

FAILED (errors=1)

The fix implemented in nested_lookup.py works for both python3 and python2.