We allow the serialization format of dates to be configured using
spring.jackson.date-format. However, this property only applies to
java.util.Date instances and has no effect on a Joda DateTime.

This commit updates our auto-configuration for Jackson to allow
the format string that is used to  serialize a Joda DateTime to be
configured. A new property, spring.jackson.joda-date-time-format has
been introduced. When configured, it is used to configure the
serialization format for a Joda DateTime. When it is not configured,
we fall back to using spring.jackson.date-format. If this fails,
either because the format string is incompatible (unlikely) or because
the user's configured the fully-qualified name of a DateFormat class,
a warning is logged encouraging the use of
spring.jackson.joda-date-time-format.

Fixes gh-2225