Gson fails to parse simple map

```
When parsing the following json with gson (using 2.2.2 from maven-central),
gson (fromJson) throws an:

JsonSyntaxException: "duplicate key: null"

whereby the json is created by gson (toJson) itself.

Json:
{"appProperties":{"server.configuration.reset":"false","server.db.jndi":"testDB"
,"xml.basedata.list":"basedata.xml","server.configuration.cache":"false","xml.la
yout.list":"","server.rolemapper":"InsertionRoleMapper","xml.content.list":"data
.xml"}}

http://json.parser.online.fr/ validates the string as correct...
```

Original issue reported on code.google.com by `phil...@meisen.net` on 15 Feb 2013 at 12:04