Allow dictionary return values as JSON

This supports an increasingly common usecase whereby JSON is the
primary response (rather than a templated string). Given Flask has a
short syntax for HTML reponses, it seems fitting that it should also
do so for JSON responses. In practice it allows,

     @app.route("/")
     def index():
         return {
             "api_stuff": "values",
         }

tests/test_basic.py

@@ -1147,8 +1147,12 @@ def test_response_types(app, client):
     def from_wsgi():
         return NotFound()
 
-    assert client.get("/text").data == u"Hällo Wörld".encode("utf-8")
-    assert client.get("/bytes").data == u"Hällo Wörld".encode("utf-8")
+    @app.route('/dict')
+    def from_dict():
+        return {"foo": "bar"}, 201
+
+    assert client.get('/text').data == u'Hällo Wörld'.encode('utf-8')
+    assert client.get('/bytes').data == u'Hällo Wörld'.encode('utf-8')
 
     rv = client.get("/full_tuple")
     assert rv.data == b"Meh"
@@ -1181,6 +1185,10 @@ def test_response_types(app, client):
     assert b"Not Found" in rv.data
     assert rv.status_code == 404
 
+    rv = client.get('/dict')
+    assert rv.json == {"foo": "bar"}
+    assert rv.status_code == 201
+
 
 def test_response_type_errors():
     app = flask.Flask(__name__)