Allow NotNullColumn to work with method calls

At the NotNullColumn if a method is passed as arguments for `add_column`
an error is trigger.
In this commit we check if the argument is a method and return it's name
as a String.

Fixes rubocop#997

lib/rubocop/cop/rails/not_null_column.rb

@@ -45,7 +45,8 @@ def on_send(node)
 
         def check_add_column(node)
           add_not_null_column?(node) do |type, pairs|
-            return if type.value == :virtual || type.value == 'virtual'
+            type_value = type.send_type? ? type.method_name.to_s : type.value
+            return if [:virtual, 'virtual'].include?(type_value)
 
             check_pairs(pairs)
           end