fix issues with method not allowed response

sanic/app.py

@@ -543,6 +543,7 @@ class Sanic:
 
                 # Fetch handler from router
                 handler, args, kwargs, uri = self.router.get(request)
+
                 request.uri_template = uri
                 if handler is None:
                     raise ServerError(

sanic/exceptions.py

@@ -150,6 +150,16 @@ class InvalidUsage(SanicException):
     pass
 
 
+@add_status_code(405)
+class MethodNotSupported(SanicException):
+    def __init__(self, message, method, allowed_methods):
+        super().__init__(message)
+        self.headers = dict()
+        self.headers["Allow"] = ", ".join(allowed_methods)
+        if method in ['HEAD', 'PATCH', 'PUT', 'DELETE']:
+            self.headers['Content-Length'] = 0
+
+
 @add_status_code(500)
 class ServerError(SanicException):
     pass

sanic/router.py

@@ -3,7 +3,7 @@ from collections import defaultdict, namedtuple
 from collections.abc import Iterable
 from functools import lru_cache
 
-from sanic.exceptions import NotFound, InvalidUsage
+from sanic.exceptions import NotFound, InvalidUsage, MethodNotSupported
 from sanic.views import CompositionView
 
 Route = namedtuple(
@@ -352,6 +352,15 @@ class Router:
         except NotFound:
             return self._get(request.path, request.method, '')
 
+    def get_supported_methods(self, url):
+        """Get a list of supported methods for a url and optional host.
+
+        :param url: URL string (including host)
+        :return: frozenset of supported methods
+        """
+        route = self.routes_all.get(url)
+        return getattr(route, 'methods', frozenset())
+
     @lru_cache(maxsize=ROUTER_CACHE_SIZE)
     def _get(self, url, method, host):
         """Get a request handler based on the URL of the request, or raises an
@@ -364,9 +373,10 @@ class Router:
         url = host + url
         # Check against known static routes
         route = self.routes_static.get(url)
-        method_not_supported = InvalidUsage(
-            'Method {} not allowed for URL {}'.format(
-                method, url), status_code=405)
+        method_not_supported = MethodNotSupported(
+            'Method {} not allowed for URL {}'.format(method, url),
+            method=method,
+            allowed_methods=self.get_supported_methods(url))
         if route:
             if route.methods and method not in route.methods:
                 raise method_not_supported