handler return automatically sets content-type

[The-EDev]

discussion in ipkn#250

Currently Crow only supports this with its own JSON data type, no xml, no anything.

one potential solution would be to create a response constructor argument that specifies the content type (as opposed to just headers), this can be done by using an extention (e.g. xml) and then using mime_types.h to get the proper content-type header.

[The-EDev]

The solution here is to add a response constructors that allow either a string containing headers or a ci_map, and a constructor with a string for Content-Type.