http/dccp: Do not send the audio file header twice.

If the http/dccp client connects before stream start and the stream
starts at the beginning of the audio file (the common case), we do not
need to send the audio file header as it is contained in chunk zero.

However, the current code fails to set sender_client->sent_header to
a non-zero value in this case. This causes the header to be sent twice.

The ogg vorbis decoder is quite forgiving in that respect, which is
probably why this bug went unnoticed for so long. This patch gets
it right.