`StdDateFormat`: serialize time offset using colon

[challarao]

I think it also applies to zone provided that the zone is a 4 or 6 digit offset.

Either basic or extended formats may be used, but both date and time must use the same format. The date expression may be calendar, week, or ordinal, and must use a complete representation. The time may be represented using a specified reduced accuracy format.[8] It is permitted to omit the 'T' character by mutual agreement.[21]

https://en.wikipedia.org/wiki/ISO_8601#Combined_date_and_time_representations

[cowtowncoder]

Note: this is manifestation of FasterXML/jackson3-dev#7 for Jackson 3.x.
For Jackson 2.x, users need to consider #1744 since we can not safely change defaults (based on past experience, seemingly small defaults changes tend to break some users somewhere).

So: change here is change of StdDateFormat to generate ISO-8601 representation, if no override specified, that does include colon separating hours and minutes in time offset.