Added tests for Collection::max
Added tests for Collection::min
Avoid writing single line statements, otherwise code coverage won't be accurated
Added tests for Collection::modelKeys
Added tests for Collection::toBase
Added tests for Collection::unique
Added tests for Collection::merge
Added tests for Collection::diff
Added tests for Collection::intersect
Fixed Collection::intersect
Added tests for Collection::__call
Check if the method exists in the current DataModel object, not the global one
Added tests for Collection::__call on an empty set
Fixed check on number of items