Pass for: to Grape::Endpoint.new for Grape HEAD compatibility

Grape HEAD introduced Grape::Endpoint::Options (a Data class) that
requires a :for keyword (the owner API class). Constructing an Endpoint
without it raises ArgumentError: missing keyword: :for.

Author: numbata

spec/lib/endpoint_spec.rb

@@ -4,7 +4,8 @@
 
 describe Grape::Endpoint do
   subject do
-    described_class.new(Grape::Util::InheritableSetting.new, path: '/', method: :get)
+    # Grape >= HEAD requires :for (the owner API class) when constructing an Endpoint.
+    described_class.new(Grape::Util::InheritableSetting.new, path: '/', method: :get, **{ for: Class.new(Grape::API) })
   end
 
   describe '.content_types_for' do