Fix svn_stream_compressed() for streams without partial read support.

* subversion/libsvn_subr/stream.c
  (read_helper_gz): Use svn_stream_read_full() to read data from substream.

* subversion/tests/libsvn_subr/stream-test.c
  (empty_read_full_fn): Helper for test_stream_compressed_read_full() test.
  (test_stream_compressed_read_full): New test.
  (test_funcs): Add test test_stream_compressed_read_full() to test list.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639626 13f79535-47bb-0310-9956-ffa450edef68