Add no_args support for stubs

Fixes https://github.com/icy-arctic-fox/spectator/issues/28

src/spectator/dsl/mocks.cr

@@ -170,4 +170,8 @@ module Spectator::DSL
     {% end %}
     %stubs
   end
+
+  def no_args
+    ::Spectator::Mocks::NoArguments.new
+  end
 end

src/spectator/mocks/no_arguments.cr

@@ -0,0 +1,21 @@
+require "./arguments"
+
+module Spectator::Mocks
+  class NoArguments < Arguments
+    def args
+      Tuple.new
+    end
+
+    def opts
+      NamedTuple.new
+    end
+
+    def ===(other : Arguments) : Bool
+      other.args.empty? && other.opts.empty?
+    end
+
+    def ===(other) : Bool
+      false
+    end
+  end
+end