improve mangle.properties (#2261)

- include dead code when `keep_quoted`
- unify `keep_quoted` & `reserved`
- make `test/run-tests.js` consistent with `minify()`

fixes #2256

test/exports.js

@@ -8,6 +8,7 @@ exports["defaults"] = defaults;
 exports["mangle_properties"] = mangle_properties;
 exports["minify"] = minify;
 exports["parse"] = parse;
+exports["reserve_quoted_keys"] = reserve_quoted_keys;
 exports["string_template"] = string_template;
 exports["tokenizer"] = tokenizer;
 exports["is_identifier"] = is_identifier;