`extractHeaders` had an incorrect regex that crashed Corsy with the msg:
"sre_constants.error: nothing to repeat at position 1". This was caused
by `^?`, which is not a valid regex, and can be reproduced with the
README example: `python3 corsy.py -u https://example.com --headers
"User-Agent: GoogleBot\nCookie: SESSION=Hacked"`.
Instead of using the regex this commit simplifies the logic by using a
`split` to split lines and header_name/value pairs.
