Filter identifier are not quantities

[bas-vk]

According to the json spec filter id's are encoded as quantities.
Quantities must be as compact as possible (apart from 0 which is 0x0).

Expected:
{"jsonrpc":"2.0","result":"0x0","id":1}
{"jsonrpc":"2.0","result":"0x1","id":1}
Got:
{"jsonrpc":"2.0","result":"0x00","id":1}
{"jsonrpc":"2.0","result":"0x01","id":1}