Add no-arg constructor for DateTimeDeserializer

[abbychau]

ref: cf2f4c2#r68702429

I have no clue if is was regarded as public API 7 years ago, but I believe so. And I would prefer to have no-arg constructor in the next version of jackson-datatype-joda too, because we have the usage like
@JsonDeserialize(using = DateTimeDeserializer::class)

here is the problem met by people: https://stackoverflow.com/questions/38433432/class-com-fasterxml-jackson-datatype-joda-deser-datetimedeserializer-has-no-defa

[cowtowncoder]

What puzzles me is that I do not actually see a no-arguments constructor in Jackson 2.6 sources either.
As far as I know, code would never have worked: that class has never had 0-args constructor as far I can see from version tags (not even in 2.0).

I will also add a note in SO question: the obvious usual answer should be "register JodaModule!".

[cowtowncoder]

Ok based on reading the question I think that this issue is actually not about fixing anything (there never was such constructor) but making it possible. That would require a different approach; if you look at JodaModule you can see the reasoning why this class has no 0-arguments constructor -- it is used as handler for 2 subtypes (ReadableDateTime, ReadableInstant). It would technically be possible to add concrete sub-classes if that was desired but not sure it's worth the effort. I am still not sure why registering JodaModule (or custom module registering the same) would not be possible.

[cowtowncoder]

Thank you @abbychau for contributing this! Will be included in 2.14.0 release.