fix: Make CollectionDecorator respond_to? to ORM methods (#850)

## Testing
1. Create a decorator for the model and its association
```ruby
class OrderHistoryDecorator < Draper::Decorator
  delegate_all
end

class OrderDecorator < Draper::Decorator
  delegate_all

  decorates_association :order_histories, with: OrderHistoryDecorator
end
```

2. Call `respond_to?` with some ORM method allowed by the strategy
```ruby
[1] pry(main)> order.order_histories.decorate.respond_to?(:includes)
=> true
```

## References
* method_missing [usage](https://thoughtbot.com/blog/always-define-respond-to-missing-when-overriding)

lib/draper/query_methods.rb

@@ -9,6 +9,10 @@ module Draper
       object.send(method, *args, &block).decorate
     end
 
+    def respond_to_missing?(method, include_private = false)
+      strategy.allowed?(method) || super
+    end
+
     private
 
     # Configures the strategy used to proxy the query methods, which defaults to `:active_record`.

spec/draper/query_methods_spec.rb

@@ -5,12 +5,13 @@ Post = Struct.new(:id) { }
 
 module Draper
   describe QueryMethods do
+    let(:fake_strategy) { instance_double(QueryMethods::LoadStrategy::ActiveRecord) }
+
+    before { allow(QueryMethods::LoadStrategy).to receive(:new).and_return(fake_strategy) }
+
     describe '#method_missing' do
       let(:collection) { [ Post.new, Post.new ] }
       let(:collection_decorator) { PostDecorator.decorate_collection(collection) }
-      let(:fake_strategy) { instance_double(QueryMethods::LoadStrategy::ActiveRecord) }
-
-      before { allow(QueryMethods::LoadStrategy).to receive(:new).and_return(fake_strategy) }
 
       context 'when strategy allows collection to call the method' do
         let(:results) { spy(:results) }
@@ -35,5 +36,28 @@ module Draper
         end
       end
     end
+
+    describe "#respond_to?" do
+      let(:collection) { [ Post.new, Post.new ] }
+      let(:collection_decorator) { PostDecorator.decorate_collection(collection) }
+
+      subject { collection_decorator.respond_to?(:some_query_method) }
+
+      context 'when strategy allows collection to call the method' do
+        before do
+          allow(fake_strategy).to receive(:allowed?).with(:some_query_method).and_return(true)
+        end
+
+        it { is_expected.to eq(true) }
+      end
+
+      context 'when strategy does not allow collection to call the method' do
+        before do
+          allow(fake_strategy).to receive(:allowed?).with(:some_query_method).and_return(false)
+        end
+
+        it { is_expected.to eq(false) }
+      end
+    end
   end
 end