wait until app ctx is ready before matching url

`RequestContext.match_request` is moved from `__init__` to `push`. This
causes matching to happen later, when the app context is available.
This enables URL converters that use things such as the database.

tests/test_converters.py

@@ -0,0 +1,38 @@
+from flask.globals import _app_ctx_stack
+
+
+def test_custom_converters(app, client):
+    from werkzeug.routing import BaseConverter
+
+    class ListConverter(BaseConverter):
+        def to_python(self, value):
+            return value.split(",")
+
+        def to_url(self, value):
+            base_to_url = super(ListConverter, self).to_url
+            return ",".join(base_to_url(x) for x in value)
+
+    app.url_map.converters["list"] = ListConverter
+
+    @app.route("/<list:args>")
+    def index(args):
+        return "|".join(args)
+
+    assert client.get("/1,2,3").data == b"1|2|3"
+
+
+def test_model_converters(app, client):
+    from werkzeug.routing import BaseConverter
+
+    class ModelConverterTester(BaseConverter):
+        def to_python(self, value):
+            assert _app_ctx_stack.top is not None
+            return value
+
+    app.url_map.converters["model"] = ModelConverterTester
+
+    @app.route("/<model:user_name>")
+    def index(user_name):
+        return user_name, 200
+
+    client.get("/admin").data