Incompatible with origin Response.raw.read

Hi!

I find that the `raw.read` would raise `TypeError` when I call `raw.read(decode_content=True)` in my project. Here is the error message:

```
TypeError: read() got an unexpected keyword argument 'decode_content'
```

The reason is that `StringIO.read` is deferent from `HTTPResponse.read` and there is no compatiblity in current code.
