Make content length available in request_info:

- given that the content-length header is already parsed, lets make
  it also available in the request_info struct to allow easy access;
- added simple unit test to verify the behavior of the new field.
Improve description of CivetServer::getParam(connection, ...)
Improve error handling in set_option
Merge pull request #34 from jawi/master

Make content length available in request_info
Fix mg_read for situations where Content-Length was not set and all data is already buffered
Adapt unit test for mg_download, so the tested http operations are standard compliant
Fix unit test for HTTPS
Improve unit test for mg_download
Fix unit test for SSL redirect
clean up unit_test: SSL related TODOs are done
Merge remote-tracking branch 'upstream/master'