Make Schema#as_json return an instance of Hash

Certain JSON encoders (namely, Oj) were previously serializing `GraphQL::Query::Result`
incorrectly. Instead of returning a schema definition, it would return serialize
the output of `GraphQL::Query::Result#inspect` as a string. In order to have better
compatibility, this patch opts to represent the schema as a Hash when calling `#as_json`.

Author: ianks

lib/graphql/schema.rb

@@ -554,7 +554,7 @@ def to_document
     # @param except [<#call(member, ctx)>]
     # @return [Hash] GraphQL result
     def as_json(only: nil, except: nil, context: {})
-      execute(Introspection::INTROSPECTION_QUERY, only: only, except: except, context: context)
+      execute(Introspection::INTROSPECTION_QUERY, only: only, except: except, context: context).to_h
     end
 
     # Returns the JSON response of {Introspection::INTROSPECTION_QUERY}.

spec/graphql/schema_spec.rb

@@ -425,6 +425,13 @@ class LazyObjChild < LazyObj; end
     end
   end
 
+  describe "#as_json" do
+    it "returns a hash" do
+      result = schema.execute(GraphQL::Introspection::INTROSPECTION_QUERY)
+      assert_equal result.as_json.class, Hash
+    end
+  end
+
   describe "#get_field" do
     it "returns fields by type or type name" do
       field = schema.get_field("Cheese", "id")