Updates the SDK's codegen for REST HTTP request payload to always
include content-type header for operations with streaming request
payloads. Prior the content-type header would only be added if the
streaming request io.Reader was not nil.

Adds a helper utility for setting if the content-type was automatically
assigned by the operation serializer or not. Used by behavior that needs
to know if an explicit or default value was provided for the
content-type header.