core: Set all headers in RygelHTTPGet

core: Set all headers in RygelHTTPGet

Don't set Content-Length header in HEAD request for transcoded
resources. This was previously caused by the message encoding being
set to CONTENT_LENGTH by default. Moving the code sets the encoding
to EOF causing libsoup to not set the Content-Length header.

Also adds a proper Range header for byte-seek HEAD requests.

This fixes the random fails of test-case 7.4.28.2,3,4,6