Use NilMethodStub by default

Provides one place that #with can be defined and enforces ordering.

src/spectator/dsl/mocks.cr

@@ -46,6 +46,6 @@ module Spectator::DSL
 
   macro receive(method_name, _source_file = __FILE__, _source_line = __LINE__)
     %source = ::Spectator::Source.new({{_source_file}}, {{_source_line}})
-    ::Spectator::Mocks::ValueMethodStub.new({{method_name.symbolize}}, %source, nil)
+    ::Spectator::Mocks::NilMethodStub.new({{method_name.symbolize}}, %source)
   end
 end

src/spectator/mocks/nil_method_stub.cr

@@ -0,0 +1,19 @@
+require "./generic_arguments"
+require "./generic_method_stub"
+require "./value_method_stub"
+
+module Spectator::Mocks
+  class NilMethodStub < GenericMethodStub(Nil)
+    def initialize(name, source, args = nil)
+      super(name, source, args)
+    end
+
+    def call(_args : GenericArguments(T, NT)) : ReturnType forall T, NT
+      nil
+    end
+
+    def and_return(value : T) forall T
+      ValueMethodStub.new(@name, @source, value)
+    end
+  end
+end

src/spectator/mocks/value_method_stub.cr

@@ -1,4 +1,4 @@
-require "./arguments"
+require "./generic_arguments"
 require "./generic_method_stub"
 
 module Spectator::Mocks
@@ -10,9 +10,5 @@ module Spectator::Mocks
     def call(_args : GenericArguments(T, NT)) : ReturnType forall T, NT
       @value
     end
-
-    def and_return(value : T) forall T
-      ValueMethodStub.new(@name, @source, value)
-    end
   end
 end