Change default setting for Date serialization to include colon

[cowtowncoder]

Jackson 2.x serializes java.util.Date and java.util.Calendar using format where timezone offset does not include colon; for example:

2017-01-04T15:00:00-0800

but Java 8 serialization and Joda add colon:

2017-01-04T15:00:00-08:00

and it sounds like latter is the way things should work wrt "basic" (or compact?) vs "extended" notations -- omitting colon would require omitting of colons and hyphens more generally.

Since Jackson uses its own StdDateFormat for serialization it is easy to change default formatting and add colon. Now (2.x -> 3.0) would be good time to do this, since it is a small but highly visible change to defaults.

[cowtowncoder]

Implemented in jackson-databind, modified tests.