split_port() does not properly handle ":80" or "127.0.0.1:" properly

[mark-adams]

Initially reported as docker/compose#1887

Example:

    def test_port_only_with_colon(self):
        self.assertRaises(ValueError,
                          lambda: split_port(":80"))

    def test_host_only_with_colon(self):
        self.assertRaises(ValueError,
                          lambda: split_port("localhost:"))

Results:

======================================================================
ERROR: test_host_only_with_colon (__main__.UtilsTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/utils_test.py", line 428, in test_host_only_with_colon
    lambda: split_port("localhost:"))
  File "/usr/lib/python2.7/unittest/case.py", line 473, in assertRaises
    callableObj(*args, **kwargs)
  File "tests/utils_test.py", line 428, in <lambda>
    lambda: split_port("localhost:"))
  File "/home/mark/Projects/docker-py/docker/utils/ports/ports.py", line 77, in split_port
    if len(internal_range) != len(external_range):
TypeError: object of type 'NoneType' has no len()

======================================================================
ERROR: test_port_only_with_colon (__main__.UtilsTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/utils_test.py", line 424, in test_port_only_with_colon
    lambda: split_port(":80"))
  File "/usr/lib/python2.7/unittest/case.py", line 473, in assertRaises
    callableObj(*args, **kwargs)
  File "tests/utils_test.py", line 424, in <lambda>
    lambda: split_port(":80"))
  File "/home/mark/Projects/docker-py/docker/utils/ports/ports.py", line 77, in split_port
    if len(internal_range) != len(external_range):
TypeError: object of type 'NoneType' has no len()