At string_with_format, fix the locale if the format is a Symbol.

If I set Region Format Japanese by Settings app, string_with_format(:iso8601) returns with Japanese style.
Maybe it should be a same value by all locales.

```
Bacon::Error: "2013年-01月-02日 12:15:30.000".==("2013-01-02 12:15:30.000") failed
```
add the timezone option to Time#strign_with_format

i.e.
```
t = Time.now
t.string_with_format('HH:mm:ss', timezone:'UTC'.nstimezone)
```
Merge pull request #133 from katsuyoshi/string_with_format

At string_with_format, fix the locale if the format is a Symbol.