"Range" CFI with singular text offset terminus step is treated as an "Element" CFI

[jccr]

1. Example Range CFI with a start and end offset: /6/14!/4,/2/14/1:4,/2/16/1:7
2. Example CFI with a single text terminus: /6/14!/4/2/14/1:4

Bug: CFI 2 is treated as /6/14!/4/2/14/1
Expected: CFI 2 should be used with the text offset

A PR with a fix is in the works by me

CFI library doesn't need changes as it provides the utilities to do this for the readium layer

[jccr]

Note to self: Verify this is fixed in upcoming PR