add parse_string test

tests/json_parser.rs

@@ -11,6 +11,16 @@ proptest! {
     fn doesnt_crash(s in "\\PC*") {
         let _output = json_object().parse(&s);
     }
+
+    #[test]
+    fn parse_string(s in r#"[^"]+"#) {
+        let input = format!("\"{}\"", s);
+        let output = json_string().parse(&input).unwrap();
+        match output {
+            (JsonValue::Str(output_s), "") if output_s == s => (),
+            _ => panic!(),
+        }
+    }
 }
 
 #[test]