add support for case insensitive key parsing

[romk1n]

Please add ability to parse json with case insensitive keys:

{ "key": "value"} should result in the same model as {"Key": "value"} and {"KEY": "value"}```.

That's especially relevant with camelcase, cause of one can write timezoneOffset and another timeZoneOffset :)

[Dynesshely]

Same feature request as for me.