fix ruby 2.7 deprecation warning (Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call)

Author: yrodnin

lib/grape_entity/entity.rb

@@ -537,7 +537,7 @@ def delegate_attribute(attribute)
       if is_defined_in_entity?(attribute)
         send(attribute)
       elsif @delegator_accepts_opts
-        delegator.delegate(attribute, self.class.delegation_opts)
+        delegator.delegate(attribute, **self.class.delegation_opts)
       else
         delegator.delegate(attribute)
       end