Return status 405 when router error was ErrMethodNotAllowed

[strowk]

I think it could be convenient impovement here. Currently 404 is coded for all router errors.

See example how it is done in another lib:
https://github.com/getkin/kin-openapi/blob/529285c16a9d45f7f908c1d61e67abcc070d192f/openapi3filter/validation_error_encoder.go#L55-L57

Place to put it here:

echo-middleware/oapi_validate.go

Lines 119 to 122 in b1361c4

	case *routers.RouteError:
	// We've got a bad request, the path requested doesn't match
	// either server, or path, or something.
	return echo.NewHTTPError(http.StatusNotFound, e.Reason)