Set DID Resolution Result media type

[clehner]

Set the Content-Type header of the DID Resolution Result HTTP response to use the media type with profile parameter specified for a DID Resolution Result in DID Resolution: application/ld+json;profile="https://w3id.org/did-resolution".

Setting this Content-Type header helps DID resolver HTTP clients detect that the response is a DID Resolution Result - rather than e.g. a DID document alone, which is also allowed by DID Resolution's HTTP(S) Binding.