JSONPointer pre-existing bug in toURIFragment()

toURIFragment() percent-encodes each reference token but never applies the
~0/~1 escaping required by RFC 6901. Since refTokens holds decoded
literal key names, any token containing ~ or / produces a URI fragment that
does not parse back to the same pointer. This should be fixed after #1078, but before the next release.

If anyone wants to work on this, ping me for more details.

Escape ~ and / in JSONPointer.toURIFragment()