HTTP Basic Auth - sending username but no password triggers exception

Reported on our repo here: https://github.com/Staffjoy/suite/issues/3

We send an API key as an HTTP basic username without a password, e.g. `curl --user API_KEY: http://app.local`. This appears to throw the following error on python 2.7 with unirest 1.1.7:

```
Traceback (most recent call last):
File "/opt/python/2.7.12/lib/python2.7/threading.py", line 801, in __bootstrap_inner
self.run()
File "/opt/python/2.7.12/lib/python2.7/threading.py", line 754, in run
self.__target(*self.__args, **self.__kwargs)
File "/home/travis/virtualenv/python2.7.12/lib/python2.7/site-packages/unirest/__init__.py", line 82, in __request
encoded_string = base64.b64encode(user + ':' + password)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
```

The traceback points to unirest/__init__.py, in __request where HTTP Basic Auth is assembled; start there and reproduce the API-key-only request described in the issue. Done means a username with no password no longer raises the reported TypeError; add or run coverage for that request if the repository provides it.

Written by the indexing model from the issue text.