Don't linkify text near a URL that looks like a URL with a bad protocol

Today I sent an email with text similar to "I can send email through smtp.gmail.com:587 or through https://www.gmail.com/" and was very surprised when Geary turned the entire sentence, from "I can" to the end of the URL, into a giant malformed link. This patch cleans up the function responsible for the error and adds a test to prevent the problem from recurring.