docs(Http): Fix incorrect documentation about cache usage.

Closes dart-archive#1066

Author: mvuksano

lib/core_dom/http.dart

@@ -415,7 +415,8 @@ class Http {
   * - xsrfHeaderName: TBI
   * - xsrfCookieName: TBI
   * - interceptors: Either a [HttpInterceptor] or a [HttpInterceptors]
-  * - cache: Boolean or [Cache].  If true, the default cache will be used.
+  * - cache: Boolean or [Cache].  If true, the default cache will be used. If false, no cache will
+  *      be used. If object of type [Cache] is provided, that object will be used as cache.
   * - timeout: deprecated
 */
   async.Future<HttpResponse> call({
@@ -468,7 +469,7 @@ class Http {
 
       if (cache == false) {
         cache = null;
-      } else if (cache == null) {
+      } else if (cache == true) {
         cache = defaults.cache;
       }