Only `DateTimeFormatter.ISO_OFFSET_DATE_TIME` accepted by `ZonedDateTimeKeyDeserializer`

[caluml]

Hello.

Is there any reason for specifying DateTimeFormatter.ISO_OFFSET_DATE_TIME as the format for parsing ZonedDateTimes in ZonedDateTimeKeyDeserializer?

This prevents valid ZonedDateTime keys from being deserialised.

If the second parameter to .parse isn't specified, then all formats supported by .parse are allowed.

I have raised a PR to change this: #305

[cowtowncoder]

I do not remember if there is any specific reason: I assume whoever added it (could have been me :) ) copied setting from non-key handling.
PR looks good, will add some notes.