Convert correctly PDFium UTF-8 integers to char/string

[stancld]

Fixes #49.

Tests:

> pytest tests                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

tests/pdf/test_chars.py ..............                                                                                                                                                                                                                                                                                                                [ 60%]
tests/test_extraction.py .......                                                                                                                                                                                                                                                                                                                      [ 91%]
tests/test_tables.py ..