Limiting the number of conversations leads to inconsistent behavior

Thanks for this great module. I noticed the following bug in privatemsg_limits.

Privatemsg settings:

• Limit the total no. of messages/conv. a user can send
• What should be limited: conversations
• Set time period: 1 week
• Maximum limit by role: role "limit2": 2

If a user sends new private messages first, they can contact two new users per week as follows:

1. User A with role limit2 sends a private message to User B.
2. User A with role limit2 sends a private message to User C.
3. User A with role limit2 replies to a private message from User D.

However, if a user replies to new private messages first, those replies reduce the number of new users they can contact:

1. User A with role limit2 replies to a private message from User D.
2. User A with role limit2 sends a private message to User B.
3. User A with role limit2 sends a private message to User C. -> This fails with a "you have exceeded your sending limit" message.

This implementation is inconsistent. A reply to a private message should not affect the limit depending on whether it is before or after the user has sent other messages.