http: simplify if statement

PR-URL: #13857
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

lib/_http_server.js

@@ -213,10 +213,8 @@ function writeHead(statusCode, reason, obj) {
         if (k) this.setHeader(k, obj[k]);
       }
     }
-    if (k === undefined) {
-      if (this._header) {
-        throw new errors.Error('ERR_HTTP_HEADERS_SENT');
-      }
+    if (k === undefined && this._header) {
+      throw new errors.Error('ERR_HTTP_HEADERS_SENT');
     }
     // only progressive api is used
     headers = this[outHeadersKey];