Fix JSONPatch.add

[jg-rp]

This PR fixes an issue with the JSON Patch add operation. Previously we would raise a JSONPatchError when pointing to an array value with an index equal to the array's length. Now we append to array values when pointing to one past the last index of the array.

To put it another way, when the target value is an array of length 2, /foo/2 is the same as /foo/-

patch = JSONPatch().add(path="/foo/2", value=99)
data = {"foo": ["bar", "baz"]}
assert patch.apply(data) == {"foo": ["bar", "baz", 99]}

# Array length + 1 raises
patch = JSONPatch().add(path="/foo/3", value=99)
data = {"foo": ["bar", "baz"]}
patch.apply(data) # JSONPatchError 

See #117