Return Accept header when Bad Content Type is reported...

[mgajda]

According to HTTP standard, when "Bad content type" is reported, we should send a list of accepted content types for the given address in Accept header.
This could be realized by a walk over the type with a given path.

This allows for content negotiation and speeds up debugging expectations of API servers:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation#the_accept_header

[mgajda]

May be related to example in #1246.