refactor($http): clean up code

Closes #14921

src/ng/http.js

@@ -959,26 +959,25 @@ function $HttpProvider() {
       config.paramSerializer = isString(config.paramSerializer) ?
           $injector.get(config.paramSerializer) : config.paramSerializer;
 
-      var chain = [serverRequest, undefined];
-      var promise = initiateOutstandingRequest(config);
+      $browser.$$incOutstandingRequestCount();
+
+      var requestInterceptors = [];
+      var responseInterceptors = [];
+      var promise = $q.when(config);
 
       // apply interceptors
       forEach(reversedInterceptors, function(interceptor) {
         if (interceptor.request || interceptor.requestError) {
-          chain.unshift(interceptor.request, interceptor.requestError);
+          requestInterceptors.unshift(interceptor.request, interceptor.requestError);
         }
         if (interceptor.response || interceptor.responseError) {
-          chain.push(interceptor.response, interceptor.responseError);
+          responseInterceptors.push(interceptor.response, interceptor.responseError);
         }
       });
 
-      while (chain.length) {
-        var thenFn = chain.shift();
-        var rejectFn = chain.shift();
-
-        promise = promise.then(thenFn, rejectFn);
-      }
-
+      promise = chainInterceptors(promise, requestInterceptors);
+      promise = promise.then(serverRequest);
+      promise = chainInterceptors(promise, responseInterceptors);
       promise.finally(completeOutstandingRequest);
 
       if (useLegacyPromise) {
@@ -1006,9 +1005,18 @@ function $HttpProvider() {
 
       return promise;
 
-      function initiateOutstandingRequest(config) {
-        $browser.$$incOutstandingRequestCount();
-        return $q.when(config);
+
+      function chainInterceptors(promise, interceptors) {
+        for (var i = 0, ii = interceptors.length; i < ii;) {
+          var thenFn = interceptors[i++];
+          var rejectFn = interceptors[i++];
+
+          promise = promise.then(thenFn, rejectFn);
+        }
+
+        interceptors.length = 0;
+
+        return promise;
       }
 
       function completeOutstandingRequest() {