Avoid using internal function `Dates.default_format`

[omus]

I noticed this deprecation warning from TimeZones.jl due to the use of Dates.default_format:

┌ Warning: `Dates.default_format(ZonedDateTime)` is deprecated and has no direct replacement. Consider using refactoring to use `parse(::Type{ZonedDateTime}, ::AbstractString)` as an alternative.
│   caller = write at write.jl:338 [inlined]
└ @ Core ~/.julia/packages/JSON3/rT1w2/src/write.jl:338

–

JSON3.jl/src/write.jl

Line 338 in cc047e0

write(::StringType, buf, pos, len, x::T; dateformat::Dates.DateFormat=Dates.default_format(T), kw...) where {T <: Dates.TimeType} = write(StringType(), buf, pos, len, Dates.format(x, dateformat); kw...)

The deprecation notice is a bit off in that we're formatting a ZonedDateTime and not parsing one.

We could fix this by either adding a special write function for ZonedDateTime or should we document that Dates.default_format should be defined for all Dates.TimeType subtypes?

[quinnj]

Yeah, defining an overload for write would probably work for now.