Move HEAD route creation into Grape::Router::Route#to_head

`Endpoint#mount_in` built the HEAD variant of a GET route inline: dup the
route, flip its request method, then apply and append it. Duplicating a
route and turning it into a HEAD request is the route's own concern, not
the endpoint's.

Add `Route#to_head`, which returns a fresh HEAD copy of the route, and
demote `convert_to_head_request!` to `protected` now that `to_head` is
its only caller. `mount_in` keeps the decision of whether to add a HEAD
route (the `do_not_route_head` setting and the GET check) and just calls
`route.to_head.apply(self)`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ericproulx

lib/grape/endpoint.rb

@@ -120,10 +120,7 @@ def mount_in(router)
         router.append(route.apply(self))
         next if inheritable_setting.namespace_inheritable[:do_not_route_head] || route.request_method != Rack::GET
 
-        route.dup.then do |head_route|
-          head_route.convert_to_head_request!
-          router.append(head_route.apply(self))
-        end
+        router.append(route.to_head.apply(self))
       end
     end
 

lib/grape/router/route.rb

@@ -19,8 +19,10 @@ def initialize(endpoint, method, pattern, options, forward_match:, **route_attri
         @match_function = forward_match ? FORWARD_MATCH_METHOD : NON_FORWARD_MATCH_METHOD
       end
 
-      def convert_to_head_request!
-        @request_method = Rack::HEAD
+      def to_head
+        head = dup
+        head.convert_to_head_request!
+        head
       end
 
       def apply(app)
@@ -43,6 +45,12 @@ def params(input = nil)
         parsed.compact.symbolize_keys
       end
 
+      protected
+
+      def convert_to_head_request!
+        @request_method = Rack::HEAD
+      end
+
       private
 
       def params_without_input