This was copied from the `argument` type, but I have to say I’m not entirely sure why it is there in the first place.  I haven’t put much thought into wether using `index` or not nor have I tested if the converters need/use it, but PHP does have a rather unusual handling or arrays, so I would be careful about removing it and using the order of the element as the `index` value. `json_{encode,decode}` does convert `array( 0 => "foo")` to `["foo"]` and it causes problems in edge scenarios.

This was added for arguments by @schmittjoh in 803dd580, perhaps he can enlighten us.