Fix weird instance where $decoded_response was actually a string.

Fix weird instance where $decoded_response was actually a string. 

I'm not 100% sure of the underlying reason but $decoded_response was being returned as a string like:

```
oauth_token=D7AfkOc7nSHfV3r4UEq5LgqV5WBISOlrjlziDWlNg&oauth_token_secret=FOP11727jWw5CTuFIjkd85mVE3GxcuMqpX0thVQ2njQ&oauth_callback_confirmed=true
```

And then because the string was not validating as empty it'd never hit ```wp_parse_args( $resp['body'] );``` and be converted into an array.

Might solve a lot of peoples issues in the forums where they can't authenticate