handle 'text/html; charset=...' as well as plain text/html

Closing, since my implementation changed rapidly.

I make it a condition, that the Content-Type starts with text/html in

Line 23 in 1b79112

accepted_content_type = content_type.startswith("text/html")

In case, that method return False, I am not further progessing on discovery in

Line 60 in 1b79112

if is_successful_response(response):

resp

Line 75 in 1b79112

if is_successful_response(response):

.
Thus, the webmention will return None.

@snarfed You may want to check, whether these changes make sense downstream for you.