Add support for parsing json response

Author: JakeDluhy

fake_xml_http_request.js

@@ -439,6 +439,12 @@
         } catch (e) {
           // Unable to parse XML - no biggie
         }
+      } else if(this.responseText && /(application\/json)|(application\/vnd\.api\+json)/.test(type)) {
+        try {
+          this.response = JSON.parse(this.responseText);
+        } catch (e) {
+          // Unable to parse JSON - no biggie
+        }
       }
 
       if (this.async) {

src/fake-xml-http-request.js

@@ -433,6 +433,12 @@ var FakeXMLHttpRequestProto = {
       } catch (e) {
         // Unable to parse XML - no biggie
       }
+    } else if(this.responseText && /(application\/json)|(application\/vnd\.api\+json)/.test(type)) {
+      try {
+        this.response = JSON.parse(this.responseText);
+      } catch (e) {
+        // Unable to parse JSON - no biggie
+      }
     }
 
     if (this.async) {

test/responding_test.js

@@ -61,6 +61,17 @@ test("does not parse the body if it's XML and another content type is set", func
   equal(xhr.responseXML, undefined);
 });
 
+test("parses the body if it's JSON and the json content type is set", function(){
+  const body = { key: 'value' };
+  xhr.respond(200, {'Content-Type':'application/json'}, JSON.stringify(body));
+  deepEqual(xhr.response, body);
+});
+
+test("does not parse the JSON body when another content type is set", function() {
+  xhr.respond(200, {'Content-Type':'application/xml'}, '{"a":"key"}');
+  equal(xhr.response, undefined);
+});
+
 test("calls the onload callback once", function(){
   var wasCalled = 0;