Search action using method argument

[genya0407]

If we define two actions with the same path but a different method, requests pointing to the path result in unintuitive behavior:

• What we expect: action with the same path and the same method is executed
• What happens: action with the same path but different method is executed
  • Sometimes, the desired action might be executed

This is because, in dispatching requests, Shelf's current implementation does not pass method to R3::Tree#match.

In order to fix this issue, I modified Dispatcher to use method in addition to path to match @tree.

[genya0407]

@katzer Thank you for reviewing my PR, and sorry for my late reply! I fixed the pointed problems. Could you check it?