Enable template auto-reloading in app.run()

When Flask app debugging is enabled (app.debug==True), and Jinja2
template auto-reloading is not explicitly disbaled, template
auto-reloading should be enabled.

If the app is instantiated, the jinja_env object is accessed (thereby
initialising the Jinja2 environment) and the server is then started with
app.run(debug=True), template auto-reloading is *not* enabled.

This is because reading the jinja_env object causes the environment
initialisation function to set auto_reload to app.debug (which isn't yet
True). Calling app.run(debug=True) should correct this in order to
remain consistent with Flask code reloading (which is enabled within
app.run() if debug == True).

flask/app.py

@@ -839,6 +839,8 @@ class Flask(_PackageBoundObject):
         options.setdefault('use_reloader', self.debug)
         options.setdefault('use_debugger', self.debug)
         options.setdefault('passthrough_errors', True)
+        if debug:
+            self.jinja_env.auto_reload = True
         try:
             run_simple(host, port, self, **options)
         finally: