Handle "estimateContentLength" on stream endpoint in subsonic api

[thelinkin3000]

Hi!

I noticed LMS doesn't handle the "estimateContentLength" parameter on the stream endpoint while using the subsonic api.

The reference for the endpoint is here http://subsonic.org/pages/api.jsp#stream

The handling of this parameter is useful for players such as VLC. Wihtout the Content-Length HTTP header set to some reasonable value, VLC does not report a current position while playing, thus the client can't report the current position nor seek to a desired position.

Note that this only happens when transcoding. If I disable transcoding on the server the error goes away and VLC reports position as expected.