Fix TypeError in ported_string when input is email.header.Header

[AndreasLF]

Fixes #148

As detailed in the linked issue, the ported_string utility crashes with a TypeError when it receives an email.header.Header object instead of a string or bytes. This behavior occurs in certain edge cases involving specific encodings or Python environment configurations.

Currently, ported_string assumes that any input that is not a string must be bytes, leading to a crash when it attempts to call str(raw_data, encoding) on a Header object.

Changes

• mailparser/utils.py: Added an explicit isinstance check for email.header.Header. Since Header objects implement __str__, they are now safely converted to string using str() before processing.
• tests/test_utils.py: Added unit test that parses a Header object to ported string, verifying that the fix works.