Allow implicit subject to take arbitrary arguments

This allows easy construction of the subject when it has an initializer
with parameters.

src/spectator/dsl/structure_dsl.cr

@@ -268,8 +268,8 @@ module Spectator::DSL
 
           # Implicit subject definition.
           # Simply creates a new instance of the described type.
-          def subject
+          def subject(*args)
-            described_class.new
+            described_class.new(*args)
           end
         {% end %}