assert on replace_separators to enforce a string

[yiannis-had]

Need that check for replace_separators

[pep8speaks]

Hello @yiannis-had! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file flatten_json/__init__.py:

Line 70:80: E501 line too long (80 > 79 characters) Line 71:80: E501 line too long (98 > 79 characters)