Fixes reply line started with 'On'

Added test and new regex fixing issue for a reply line starting with 'On'. The regex was so greedy it would continue down to the next instance of 'wrote:'. 

Matches up with a recent submission to the Github email_reply_parser: https://github.com/danielchatfield/email_reply_parser/commit/c7678f09848ef334cc352c5b875371f844927910

From that patch: "This is the fix to #23. It uses the negative lookahead assertion to prevent matches where On appears twice. The whitespace matcher after the second on is required to make sure that people with names beginning with 'On' don't trigger it."