Detect body parameter to implement #89

We've already got a check for `bodyParams.Count > 1` (line 429), then one for `bodyParams.Count == 1` (line 434), and both of these throw or return if they are matched. 

We don't need to check explicitly for `bodyParams.Count == 0` on line 440, since it can't be anything else. (Unless I'm missing something - I am a little slow today :beers:)