Fix strings with HTML causing tox-prpl to send more than intended

When sending a string containing characters which have HTML escape sequences, tox-prpl assigns too much memory. This issue fixes something like the following situation from happening (can't remember the exact length, it's irrelevant):

Send: Yes) I agree
Receive: Yes) I agree Yes)

Also, I'm sorry for not having put #39, #40 and this pull request together, I still need to learn branches a bit more :)