Update MembershipHelper.cs

The parameter "logMemberIn" wasn't used so setting "LoginOnSuccess" on the RegisterModel didn't have any effect. By checking the parameter you can disable automatically logging in the user after registration. If registration failed, it would have returned null in an earlier stage so it's safe to say registration succeeded at this point.
Merge pull request #524 from Odie20XX/patch-1

Update MembershipHelper.cs