soup-body-output-stream: fix closing of chunked stream

When closing a chunked stream, we need to write the final 0-length
chunk. However, we can only do that if we successfully wrote the
previous chunk.

https://bugzilla.gnome.org/show_bug.cgi?id=703297
soup-filter-input-stream: fix read_until() with subclasses

If doing a read_line()/read_until() in a subclass, we need to use the
subclass's read implementation rather than just going directly to the
base stream.
chunk-io-test: new test of the chunked I/O read/write streams