The /api/security/users/<username> API returns false information in the Admin field

If a user was granted with admin privileges from a group (i.e. let's say shlomi is part of a group named Product that is configured with Admin privileges), when executing GET /api/security/users/<username>

You will get the following response: 

{
 "name" : "shlomi",
 "email" : "a@a.a",
 "admin" : false,
 "profileUpdatable" : true,
 "internalPasswordDisabled" : false,
 "groups" : [ "bla" ],
 "lastLoggedInMillis" : 0,
 "realm" : "internal",
 "offlineMode" : false,
 "disableUIAccess" : false
}

You can see that "admin" : false while the user is an admin user. 

We should fix it so if a user got admin privileges from one or more groups he is associated with, the admin flag should be true.