fix: Ensure plain text response is only used for strings

[mattbearman]

It turns out when expected errors occur (eg: those defined via potential_error in an endpoint) they are still handled via the response rack_triplet method. This means the error hash was being treated as plain text instead of JSON if the endpoint specified a plain text response

This PR fixes that by only rendering a plain text response if the type is plain text and the response body is a string

[jimehk]

I don't think it'll cause any issues, but technically a body object that is an array of strings, or really anything that responds to #each is a valid body value according to Rack.

This this is specifically for the sake of errors, it might be cleaner if we could used some other indicator than the class of the body object to determine what to do.

[mattbearman]

@jimehk true, although in terms of how Apia uses Rack responses it won't be an issue, as we only ever send hashes or strings.

It would definitely be good to make it a bit more robust in this sense, but as Apia was originally built for JSON responses only, it'll take quite a bit of work to make it more response-type agnostic. Definitely something to keep in mind for a future Apia update though