Added some test cases to cover some edge cases

[hiranya911]

Latest test coverage results after this change:

---------- coverage: platform linux2, python 2.7.6-final-0 -----------
Name                             Stmts   Miss  Cover
----------------------------------------------------
firebase_admin/__init__.py          94      2    98%
firebase_admin/_http_client.py      40      1    98%
firebase_admin/_user_mgt.py        157      7    96%
firebase_admin/_utils.py            14      2    86%
firebase_admin/auth.py             219     11    95%
firebase_admin/credentials.py       78      1    99%
firebase_admin/db.py               315     10    97%
firebase_admin/storage.py           26      2    92%
----------------------------------------------------
TOTAL                              943     36    96%