http: avoid duplicate isArray()

PR-URL: https://github.com/nodejs/node/pull/10654
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

lib/_http_client.js

@@ -122,7 +122,8 @@ function ClientRequest(options, cb) {
     self.once('response', cb);
   }
 
-  if (!Array.isArray(options.headers)) {
+  var headersArray = Array.isArray(options.headers);
+  if (!headersArray) {
     if (options.headers) {
       var keys = Object.keys(options.headers);
       for (var i = 0, l = keys.length; i < l; i++) {
@@ -166,7 +167,7 @@ function ClientRequest(options, cb) {
     self.useChunkedEncodingByDefault = true;
   }
 
-  if (Array.isArray(options.headers)) {
+  if (headersArray) {
     self._storeHeader(self.method + ' ' + self.path + ' HTTP/1.1\r\n',
                       options.headers);
   } else if (self.getHeader('expect')) {