Rebase on master

- Cleaned up fix to prevent additional nesting
- Added unit test

Fixes error:

```
>               splits = [p.partition(b"=") for p in request.body.split(b"&")]
E               AttributeError: 'NoneType' object has no attribute 'split'
```

vcr/filters.py

@@ -82,6 +82,10 @@ def replace_post_data_parameters(request, replacements):
       3. A callable which accepts (key, value, request) and returns a string
          value or None.
     """
+    if not request.body:
+        # Nothing to replace
+        return request
+
     replacements = dict(replacements)
     if request.method == "POST" and not isinstance(request.body, BytesIO):
         if request.headers.get("Content-Type") == "application/json":