http: improve invalid header character error

This commit includes the header name in the error message when invalid
characters are in the value.

Author: evanlucas

lib/_http_outgoing.js

@@ -355,7 +355,8 @@ OutgoingMessage.prototype.setHeader = function(name, value) {
   if (this._header)
     throw new Error('Can\'t set headers after they are sent.');
   if (common._checkInvalidHeaderChar(value) === true) {
-    throw new TypeError('The header content contains invalid characters');
+    throw new TypeError(`The header content for ${JSON.stringify(name)} ` +
+                        'contains invalid characters');
   }
   if (this._headers === null)
     this._headers = {};

test/parallel/test-http-client-invalid-header.js

@@ -0,0 +1,14 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+assert.throws(function() {
+  // Header value with CRLF should throw
+  http.get({
+    path: '/',
+    headers: {
+      a: 'bad value\r\n'
+    }
+  }, common.fail);
+}, /header content for "a" contains invalid characters/);