core: Don't duplicate the received chunks in GDataDownloadStream
core: Don't duplicate the received chunks in GDataDownloadStream

We were accumulating SoupMessage::got-chunk connections whenever the
network thread was restarted. This meant that we were pushing the same
chunks of data multiple times into the GDataBuffer.

https://bugzilla.gnome.org/show_bug.cgi?id=755976

pwithnall/libgdata,GNOME/libgdata,pwithnall/libgdata