JSON Pointer should escape /,~ to ~1,~0

[kuboon]

https://www.rfc-editor.org/rfc/rfc6901

Because the characters '~' (%x7E) and '/' (%x2F) have special meanings in JSON Pointer, '~' needs to be encoded as '~0' and '/' needs to be encoded as '~1' when these characters appear in a reference token.

This patch will fix compatibility with other json pointer gems like https://github.com/tenderlove/hana