close server if parsing of header field failed

Seen in #828011 if we fail to parse a header field like Last-Modified we
end up interpreting the data as response header for coming requests in
case we don't rotate to a new server in DNS rotation.

methods/server.cc

@@ -556,6 +556,7 @@ int ServerMethod::Loop()
 	 {
 	    _error->Error(_("Bad header data"));
 	    Fail(true);
+	    Server->Close();
 	    RotateDNS();
 	    continue;
 	 }