request_headers is usually an array, so use a plural variable name
Added comments for request_headers
Created the class apache::mod::headers to specifically handle mod_headers. This will automatically be enabled if the request_headers parameter is defined when calling apache::vhost
_requestheader.erb now rejects empty arrays, and skips empty strings in arrays.

an array of empty strings ['',''] will still include the fragment, but no RequestHeader rules, which should be safe.