[Http4s] Content-Type conflict

[yisraelU]

An Issue arises when there is a conflict between the media type defined in the CodecApi and a member of a struct that has a httpHeader trait setting the Content-Type on it.

• This is a problem in the StringAndBlobSchemaVisitor which allows the MediaType to be dynamic and derived from the schema
  For example:

structure MyStructure {
    @httpHeader("Content-Type")
    contentType: String,

    @httpPayload
    data: Blob
}

This is caused by setting the content type using the EntityEncoder from http4s withContentType .

• This is also a problem in simpleRestJson , as it causes the httpHeader trait to be ignored when using it for Content-Type
• Aso I would add that changing the order of the arguments to combine method , would change this behavior which did take me by surprise

[kubukoz]

Combine was never commutative I think, if you have functions that override the same field you can't just compose them in both directions

[yisraelU]

edited explanation of what surprised me

[kubukoz]

related: #28 #29

[yisraelU]

fixed