Convert object keys to numbers

[boomfly]

I am making library for Bluez DBus. ManufacturerData that format: a{qv} But when passing that object:

{
  0x0059: {...}
}

0x0059 key was converting to string and produces wrong company id two bytes

[alexcastillo]

Hi,

This PR will allow us to use this library. Any chance this can be reviewed/merged?

Thanks!