Cache Common Header Values.

[harshgMSFT]

For Input and output formatters the supported media types should come from a cached pool. #868 , #896

[rynowak]

Consider just making a static class with readonly instances on it and supporting instance of MediaTypeHeader in the ProducesAttribute.

Then you can write code like this:

[HttpGet("{id}")]
[Produces(MediaTypes.ApplicationJson, MediaTypes.ApplicationXml, Type = typeof(Product))]
public Product GetProduct(int id)
{
}

As a bonus our comparison for media types will generally just be a fast-path reference equality test.

[danroth27]

Agree with @rynowak.

[yishaigalatzer]

@Tratcher do you think this kind of class belongs in HttpAbstractions for common Content types?

[Tratcher]

I hesitate to provide statics like that because the header types like MediaTypeHeaderValue are mutable.

[yishaigalatzer]

Depends on aspnet/HttpAbstractions#177

[rynowak]

Closing this out after some discussion. The example provided above can't work because a MediaTypeHeaderValue can't be const, and so can't be used in an attribute.

If we have a perf issue, we'll drive it another way.