Merge pull request #1583 from testfirstcoder/fix-possible-nullreferen…

…ceexception

use static equals method to avoid exception

src/IdentityServer/Test/TestUserStore.cs

@@ -44,7 +44,7 @@ public bool ValidateCredentials(string username, string password)
                 return true;
             }
 
-            return user.Password.Equals(password);
+            return Equals(user.Password, password);
         }
 
         return false;