Use boolean json representation for obscureText (#9927)

Corrects type from string introduced in
21712a49.

packages/flutter/lib/src/services/text_input.dart

@@ -66,7 +66,7 @@ class TextInputConfiguration {
   Map<String, dynamic> toJSON() {
     return <String, dynamic>{
       'inputType': inputType.toString(),
-      'obscureText': obscureText.toString(),
+      'obscureText': obscureText,
       'actionLabel': actionLabel,
     };
   }

packages/flutter/test/services/text_input_test.dart

@@ -22,7 +22,7 @@ void main() {
       );
       final Map<String, dynamic> json = configuration.toJSON();
       expect(json['inputType'], 'TextInputType.number');
-      expect(json['obscureText'], 'true');
+      expect(json['obscureText'], true);
       expect(json['actionLabel'], 'xyzzy');
     });
   });