Add Entity.[] for Grape >= 3.2 param type compatibility

Grape 3.2 requires all param types to respond to [] for coercion.
Entity classes used as param types (e.g. `type: UserEntity`) crash
at route definition time because Grape::DryTypes checks
respond_to?(:[]) and raises ArgumentError when it is missing.

Add a pass-through [] class method that satisfies the check without
altering any existing behavior.

Author: numbata

lib/grape_entity/entity.rb

@@ -129,6 +129,12 @@ def hash_access=(value)
       def delegation_opts
         @delegation_opts ||= { hash_access: hash_access }
       end
+
+      # Satisfies the respond_to?(:[]) check in Grape::DryTypes (>= 3.2)
+      # so Entity subclasses can be used as param types.
+      def [](val)
+        val
+      end
     end
 
     @formatters = {}

spec/grape_entity/entity_spec.rb

@@ -1010,6 +1010,22 @@ class Parent < Person
       end
     end
 
+    describe '.[]' do
+      it 'returns the input unchanged' do
+        hash = { name: 'Test' }
+        expect(subject[hash]).to eq(hash)
+      end
+
+      it 'returns nil unchanged' do
+        expect(subject[nil]).to be_nil
+      end
+
+      it 'is inherited by subclasses' do
+        subclass = Class.new(subject)
+        expect(subclass[{ id: 1 }]).to eq(id: 1)
+      end
+    end
+
     describe '.represent' do
       it 'returns a single entity if called with one object' do
         expect(subject.represent(Object.new)).to be_kind_of(subject)