ValueConverter: Improve numeric objkey detection
ValueConverter: Improve numeric objkey detection

- The previous code threw if the first key was numeric rather than
  associative, which could give some false positives when objects use
  ONLY numeric keys. The new code instead looks for an `int(0)` key as
  the first key, and if so it checks the rest of the keys and only
  rejects the input if it's TRULY a plain, numeric array (with
  sequential keys) rather than an object key-value array.

SteveJobzniak/LazyJsonMapper,SteveJobzniak/LazyJsonMapper,aitte2/LazyJsonMapper,aitte2/LazyJsonMapper