Added a parameter to add RequestHeader declarations to apache::vhost

I've added a requestheader parameter to the vhost declaration that takes a string or and array of string and creates a bunch of RequestHeader declarations in the virtual host configuration file.

More details on what those declratations should be here: http://httpd.apache.org/docs/2.2/mod/mod_headers.html#requestheader

This patch is required when making a vhost configuration for puppetmaster under `apache::mod::passenger`