Skip 100 Continue response header
Always use a temp file when posting data

There was an odd hang when posting by sending the data directly to the
process that did not manifest when writing the data to a file and
sending it through that way.  The code is a bit more involved,
especially with cleanup of the written file, but now curl behaves as
expected.
Update tests to reflect changes
Merge branch 'issue-4-multipart-headers'
Version 0.5.3