Avoid setting priority if it is the same
Avoid setting priority if it is the same

I got runtime exceptions when the priority was changed from 1000 to
1000. This is likely a bug in the NSLayoutConstraint class, but this
workaround is safe.

gaurav1981/KeepLayout,K-Be/KeepLayout,Tricertops/KeepLayout,K-Be/KeepLayout,iMartinKiss/KeepLayout,Tricertops/KeepLayout