Simple patch for a crash that occurs when yaws cannot find a file's mime-type.

=ERROR REPORT==== 21-Feb-2013::12:02:48 ===
Yaws process died: {{badmatch,[]},
                    [{yaws,make_expires_header,2,
                           [{file,"src/yaws.erl"},{line,1460}]},
                     {yaws,outh_serialize,0,
                           [{file,"src/yaws.erl"},{line,1647}]},
                     {yaws_server,deliver_accumulated,4,
                                  [{file,"src/yaws_server.erl"},{line,3722}]},
                     {yaws_server,handle_ut,4,
                                  [{file,"src/yaws_server.erl"},{line,2188}]},
                     {yaws_server,aloop,4,
                                  [{file,"src/yaws_server.erl"},{line,1175}]},
                     {yaws_server,acceptor0,2,
                                  [{file,"src/yaws_server.erl"},{line,1016}]},
                     {proc_lib,init_p_do_apply,3,
                               [{file,"proc_lib.erl"},{line,227}]}]}

The crash occurs when Yaws can't find a corresponding mime-type