Fix bug when validating schemas that specify an explicit metaschema

Schema.assert_valid_schema/1 was checking whether an input schema was one of the JSON Schema draft metaschemas, and if so, automatically marking it as valid.  The problem, though, is that it was checking the $schema field instead of the $id field.  This meant that any schema which specified that it's supposed to be checked against a JSON Schema draft would automatically pass even if it wasn't valid.

lib/ex_json_schema/schema.ex

@@ -373,13 +373,13 @@ defmodule ExJsonSchema.Schema do
     end)
   end
 
-  defp meta04?(%{"$schema" => @draft4_schema_url <> _}), do: true
+  defp meta04?(%{"$id" => @draft4_schema_url <> _}), do: true
   defp meta04?(_), do: false
 
-  defp meta06?(%{"$schema" => @draft6_schema_url <> _}), do: true
+  defp meta06?(%{"$id" => @draft6_schema_url <> _}), do: true
   defp meta06?(_), do: false
 
-  defp meta07?(%{"$schema" => @draft7_schema_url <> _}), do: true
+  defp meta07?(%{"$id" => @draft7_schema_url <> _}), do: true
   defp meta07?(_), do: false
 
   defp do_get_fragment(nil, _, _ref), do: {:error, :invalid_reference}

test/ex_json_schema/schema_test.exs

@@ -33,10 +33,18 @@ defmodule ExJsonSchema.SchemaTest do
 
   test "schema is validated against its meta-schema" do
     schema = %{"properties" => "foo"}
+    schema_meta_draft7 = %{
+      "$schema" => "http://json-schema.org/draft-07/schema#",
+      "properties" => "foo"
+    }
 
     assert_raise ExJsonSchema.Schema.InvalidSchemaError,
                  ~s(schema did not pass validation against its meta-schema: [%ExJsonSchema.Validator.Error{error: %ExJsonSchema.Validator.Error.Type{actual: "string", expected: ["object"]}, path: "#/properties"}]),
                  fn -> resolve(schema) end
+
+    assert_raise ExJsonSchema.Schema.InvalidSchemaError,
+                 ~s(schema did not pass validation against its meta-schema: [%ExJsonSchema.Validator.Error{error: %ExJsonSchema.Validator.Error.Type{actual: "string", expected: ["object"]}, path: "#/properties"}]),
+                 fn -> resolve(schema_meta_draft7) end
   end
 
   test "resolving an absolute reference in a scoped schema" do