Member level changes to donation level for donation-only level.

[ipokkel]

Describe the bug
If a user already has a membership level and donates by checking out for a donation-only level their level is changed to the donation-only level if the donation-only level is inside the same level group that only allows a user to have one membership level in that group.

To Reproduce
Steps to reproduce the behavior:

1. Go to Memberships > Settings > Levels, create a new level group that allows users to only select one level from this group.
2. Add a free level and a donations-only level to this group.
3. Check out for the free level.
4. Check out for the donation-only level
5. See on the membership account page that the user's level changed from the free level to the donations-only level.

Expected behavior
When a user checks out for a donation-only level their original level should not be replaced with the donation-only level.

Isolating the problem (mark completed items with an [x]):

• I have deactivated other plugins and confirmed this bug occurs when only Paid Memberships Pro plugin is active.
• This bug happens with a default WordPress theme active, or Memberlite.
• I can reproduce this bug consistently using the steps above.

WordPress Environment

Details

``` PMPro 3.0.3 ```

[andrewlimaza]

I have tried to replicate this, I haven't been able to replicate the issue. @ipokkel please can you run another test and confirm this for me, I tested with PayPal Express.