toJson converts map's Integer key into a String

What steps will reproduce the problem?
1. Construct a Map with Integer key
2. Put key/value pair into the map, making sure the key is an Integer
3. Write JSON stream using toJson to convert the map to JSON
4. Check the output file

What is the expected output? What do you see instead?
Expect to see the key represented as an Integer with no quotes. The key is 
represented as a String with double quotes.

What version of the product are you using? On what operating system?
Goobuntu. The latest in third_party/java.

Please provide any additional information below.
Might have the same root cause as issue 524, but I'm seeing the problem in 
toJson instead of fromJson.

Here's some sample code to demonstrate the bug:

private void writeJsonStreamTest(OutputStream out) throws IOException {
  JsonWriter writer = new JsonWriter(new OutputStreamWriter(out, "UTF-8"));
  writer.setIndent("  ");
  writer.beginArray();
  Map<Integer, String> data = new HashMap<Integer, String>();
  data.put(new Integer(4), "Hello");
  new Gson().toJson(data, HashMap.class, writer);
  writer.endArray();
  writer.close();
}

Here's the actual output from calling that method:
[
  {
    "4": "Hello"
  }
]

Expected this instead:
[
  {
    4: "Hello"
  }
]

Original issue reported on code.google.com by [e-mail] on 19 Aug 2014 at 5:29

[abatkin]

For reference, RFC4627 makes clear:

An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

(emphasis mine)

[GoogleCodeExporter]

"new Gson().toJson(data, new TypeToken<Map<Integer, String>>() {}.getType())"
The above method is not work.

Original comment by [e-mail] on 31 Dec 2014 at 4:15