PasswordBox: validationTooltip bugfix

PasswordBox has the same bug as TextBox and ComboBox had, this should fix it.

https://github.com/MahApps/MahApps.Metro/issues/134
Merge pull request #425 from stefan-schweiger/patch-1

PasswordBox: validationTooltip bugfix
Merge branch 'master' of github.com:MahApps/MahApps.Metro