Issue title: NSDate always nil
 Issue body: Date values are always nil...

When using this:
let json = JSON(item) as JSON

Item looks like this:
dict before json: {
    date = "2015-04-15 12:00:00 +0000";
    details = "bla bla bla";
    id = 363;
    name = "Name";
}

After that date is always nil...

 Comments: 
 Comment 0: Can you paste in the code of how you're getting this date value?

 Comment 1: Feel free to reopen this if you have any more questions.
{'source': 'GithubIssues', 'repo_name_with_owner': 'SwiftyJSON/SwiftyJSON', 'issue_id': 101596188, 'repo_stars': 21607.0}