Don't cache responses in FailoverUri.

Issue #184

Change-Id: If7137f96ac4994d593615ef6baecadb668218403

lib/util/failover_uri.js

@@ -97,7 +97,7 @@ shaka.util.FailoverUri.resolve = function(baseUrl, url) {
 shaka.util.FailoverUri.prototype.fetch =
     function(opt_parameters, opt_estimator) {
   if (this.requestPromise_) {
-    // A fetch has already completed or is in progress.
+    // A fetch is already in progress.
     return this.requestPromise_;
   }
 
@@ -153,6 +153,9 @@ shaka.util.FailoverUri.prototype.createRequest_ =
   var p = this.request_.send().then(shaka.util.TypedBind(this,
       /** @param {!XMLHttpRequest} xhr */
       function(xhr) {
+        // |requestPromise_| MUST be set to null; otherwise, we will
+        // effectively cache every response.
+        this.requestPromise_ = null;
         this.request_ = null;
         return Promise.resolve(xhr.response);
       }));