Revision 1340989

AVRO-1087. C: avro_file_writer_open() and appending values works correctly.

The avro_file_writer_open() function wasn't correctly setting up the
avro_file_writer_t instance to allow you to append data to an existing
file.  We now open up the file for read+write, read in the header, and
set up the writer's codec to match what's in the existing file.  The
patch includes a test case.

Contributed by Pugachev Maxim.