okcurl truncates values of headers with timestamp values

Fixes an issue where the full timestamp is not sent to server.

Example
./target/okcurl-2.0.1-SNAPSHOT-jar-with-dependencies.jar --header 'If-Modified-Since: Thu, 04 Oct 2013 09:28:58 GMT' -i example.com

would submit: [If-Modified-Since] => Thu, 04 Oct 2013 09