Add json support for application/problem+json content type

[dennispassway]

I have several projects that return a application/problem+json content type. As seen in this RFC, it should always be possible to parse it with response.json().

The symfony framework uses this as default, so when my back-end is build with symfony I cannot select items in the body of the error at this moment since it is parsed as text.

This PR adds support for the application/problem+json content type so that the errors in json format are parsed as json instead of text.