fix: fix the fix to #884 to fix #1025

[dhdaines]

The fix in #885 was incorrect, because it caused tokens straddling a buffer boundary to be split in half, leading to #1025. This fixes #1025 (and also #885)

The code is somewhat fragile, particularly the EOF handling. It would make a lot more sense just to read the entire stream into memory and treat EOF as a valid end of token...

How Has This Been Tested?

A new unit test with the specific cmap stream mentioned in #1025

Checklist

• [x ] I have read CONTRIBUTING.md.
• [x ] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x ] I have tested that this fix is effective or that this feature works.
• [x ] I have added docstrings to newly created methods and classes.
• [x ] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.

[dhdaines]

This is also fixed by #1041 which also improves speed and correctness, so feel free to close it.