Fix for https://github.com/gabrielfalcao/HTTPretty/issues/188

The following code fixes 
https://github.com/gabrielfalcao/HTTPretty/issues/188
Basically unix socket connect calls get string - the path to file as an address
I think [isinstance](https://docs.python.org/3/library/functions.html?highlight=isinstance#isinstance) is a better solution for this:
```python
if not isinstance(address, tuple):  # a string unix socket file name
```

You could also consider checking it it's an instance of ```basestring``` instead of not a tuple.