BUG: JsonPointer indexes into strings (RFC 6901 non-compliant)

[angela-tarantula]

JsonPointer incorrectly treats JSON strings as indexable sequences.

>>> from jsonpointer import JsonPointer
>>> JsonPointer("/foo/0").resolve({"foo": "should-not-be-indexable"})
's'

Per RFC 6901, reference tokens only apply to objects (member lookup) and arrays (index lookup). Applying /0 to a string should fail, but it returns the first character instead.

This bug has downstream implications as well:

>>> from jsonpatch import JsonPointer
>>> patch = [{'op': 'test', 'path': '/foo/0', "value": "s"}]
>>> doc = {"foo": "should-not-be-indexable"}
>>> JsonPatch(patch).apply(doc)
{'foo': 'should-not-be-indexable'}

>>> from jsonpatch import JsonPointer
>>> doc = {"foo": "should-not-be-indexable"}
>>> patch = [{'op': 'copy', 'from': '/foo/0', "path": "/bar"}]
>>> JsonPatch(patch).apply(doc)
{'foo': 'should-not-be-indexable', 'bar': 's'}