[Core] - Content header writer refactor ,add content header test, and remove mediaTypeWriters

[yisraelU]

This PR changes how the content-type header is added to an outbound request in the Client codecs.

Currently , the mediaTypeWriters is invoked only when the MetadataEncoders are set .
The default for the UnaryClientCodecs is to have the Metadata Encoders set to None . ( I assume this is because some protocols dont have metadata ) . I observed when creating a test for the Content-Type header , if I did not set the metadata encoder a content type header would not be added . This is a hidden dependency and is incorrect behavior . The source for this logic is here , where if the metadata encoders field is None , we just return the mediaTypeWriters , which does not know how to decompose a schema and partition out the body part

The logic for setting content-type header is ,

• there must be content i.e. it should not be set if the body is empty
• if rawStringsAndBlobs is set and no mediaType is set , content-type is set according to the default media-type , otherwise use the mediaType value
• default to the protocol configured by the requestedMediaType setting

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Updated changelog

[lewisjkl]

I think this looks good, just clarifying question on the one test

[lewisjkl]

Not sure I understand what is meant by this, can you elaborate?

[yisraelU]

My apologies , this is leftover comments from a completely different attempt at solving the underlying issue

[yisraelU]

setting back to draft as I need to clarify some additional points