Signing broke with parameters that have a None value

[samuraisam]

The following payload correctly signs

ret = sesh.put('/users/me', {'username': 'myusername', 'email': 'myemail@test.net'})

However, if one of your parameters is None it does not correctly sign:

ret = sesh.put('/users/me', {'username': None, 'email': 'myemail@test.net'})

The issue is that it tries to sign with the string "username=None" however it should ignore that parameter (as the requests library does). The requests library does not sign "None" paramters in POST bodies.