app: Use _find_attr to locate controller's allowed_methods

Using the `milla.app.Application._find_attr` method to find the
`allowed_methods` of returned controller callables allows the attribute to be
defined e.g. on the class for instance methods, etc.

src/milla/app.py

@@ -75,8 +75,10 @@ class Application(object):
         if request.method == 'POST' and '_method' in request.POST:
             request.method = request.POST.pop('_method')
 
-        allowed_methods = getattr(func, 'allowed_methods',
-                                  self.DEFAULT_ALLOWED_METHODS)
+        try:
+            allowed_methods = self._find_attr(func, 'allowed_methods')
+        except AttributeError:
+            allowed_methods = self.DEFAULT_ALLOWED_METHODS
         if request.method not in allowed_methods:
             allow_header = {'Allow': ', '.join(allowed_methods)}
             if request.method == 'OPTIONS':