Add processing of Accept header to AudioServlet

[J-N-K]

Closes #1350

This checks the Accept header of the incoming request and adjusts the content-type (if possible). This is the best we can do to support clients that need a non-standard MIME type.

Signed-off-by: Jan N. Klug github@klug.nrw

[lolodomo]

Code looks good to me.

What could have be done too is to add an additional serve method with a new parameter to provide the MINE type to consider when the audio servlet is returning data. Like that, the squeezebox binding in its audio sink could have set it here:
https://github.com/openhab/openhab-addons/blob/da59cdd255a66275dd7ae11dd294fedca4942d30/bundles/org.openhab.binding.squeezebox/src/main/java/org/openhab/binding/squeezebox/internal/SqueezeBoxAudioSink.java#L96
Edit: to clarify, it would give the opportunity to a binding to order the openHAB audio server what MINE type to return for a particular file it serves.
WDYT ?

[J-N-K]

Why would one want to do that? The possible MIME types can be determined from Accept header (client side) and the stream.getFormat (server-side). Maybe a mis-match between those two (i.e. the determined MIME type is not present in the Accept-header should result in a 406 error.

[lolodomo]

You're probably right.
I was thinking about the strange case where the client was not providing any Accept header. I don't know if the software of the Squeezebox is providing one.

[kaikreuzer]

lgtm, thanks!