http: correct error message for invalid trailer

Prevent misleading error messages when trailers are invalid.

PR-URL: https://github.com/nodejs/node/pull/6308
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Brian White <mscdex@mscdex.net>

lib/_http_outgoing.js

@@ -519,7 +519,7 @@ OutgoingMessage.prototype.addTrailers = function(headers) {
         'Trailer name must be a valid HTTP Token ["' + field + '"]');
     }
     if (common._checkInvalidHeaderChar(value) === true) {
-      throw new TypeError('The header content contains invalid characters');
+      throw new TypeError('The trailer content contains invalid characters');
     }
     this._trailer += field + ': ' + escapeHeaderValue(value) + CRLF;
   }