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
#U4-5738 Fixed Due in version 7.2.0

Umbraco 7.2 installation via NuGet fails
Move/remove row button hover wasn't correct, point on control's methods instead of row's
Merge branch '7.2.0' of https://github.com/umbraco/Umbraco-CMS into 7.2.0
Init as initial
Optional Custom Row/Control setting