Updated list_servers signature at Mock.

lib/fog/digitalocean/requests/compute_v2/list_servers.rb

@@ -13,7 +13,7 @@ module Fog
 
       # noinspection RubyStringKeysInHashInspection
       class Mock
-        def list_servers
+        def list_servers(filters = {})
           response = Excon::Response.new
           response.status = 200
           response.body = {
@@ -25,4 +25,4 @@ module Fog
       end
     end
   end
-end
+end