Define HeaderDelegate to be a provider

Nothing in the JAX-RS 2.1 spec says that javax.ws.rs.ext.RuntimeDelegate$HeaderDelegate is a provider, which means that there is no reason to expect that something like javax.ws.rs.core.Configurable.register(Class) will work for a HeaderDelegate. Moreover, there is no compulsion to recognize a declaration of a HeaderDelegate in a META-INF/services/javax.ws.rs.ext.Providers file, or a definition of a HeaderDelegate annotated with @Provider.

It's much easier to register a HeaderDelegate if it is recognized as a provider. In particular, it would permit declarative registration. Moreover, just as one might want to use an application specific MessageBodyReader, one might want to use an application specific header, so it makes sense to think of a HeaderDelegate as a provider.

Start by reviewing the JAX-RS 2.1 specification sections covering RuntimeDelegate.HeaderDelegate, Configurable.register(Class), META-INF/services/javax.ws.rs.ext.Providers, and @Provider. Done should be a decided specification change that clearly defines whether HeaderDelegate is a provider and how its declarative and programmatic registration are recognized.

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