Consider adding support for returning more than just body from responseBodyBuilder

[samandmoore]

i'm musing about something like this

// return this, no processing
return CharlatanResponse.raw(
  body: '{"id": 12}', // already a string
  headers: {'content-type': 'application/json'},
  statusCode: 200,
);

// return this but process the body
return CharlatanResponse(
  // still a map to be converted automatically based on the incoming accept header
  // or defaulting to json
  {'id': 12},
  statusCode: 200,
);

// current behavior would still work
return {'id': 12}; 

the reason i'm curious about this approach is that it would be nice to have a way to dynamically decide on the status code in addition to the response body. e.g. sometimes you might wanna do a 422 and not a 200.

[samandmoore]

resolved in #5