Resolves rspec/rspec-core#950 and adds specs

* Hopefully the code clarity is improved here
* We also change the behavior by *not* duping an object if `space` is
  not yet initialized. This should have been the behavior from the
  beginning, but it was overlooked and did not have specs.

Author: alindeman

lib/rspec/mocks/extensions/marshal.rb

@@ -1,13 +1,13 @@
 module Marshal
   class << self
-    def dump_with_mocks(*args)
-      object = args.shift
-
-      if ( ::RSpec::Mocks.space && !::RSpec::Mocks.space.registered?(object) ) || NilClass === object
-        return dump_without_mocks(*args.unshift(object))
+    # Duplicates any mock objects before serialization. Otherwise,
+    # serialization will fail because methods exist on the singleton class.
+    def dump_with_mocks(object, *rest)
+      if !::RSpec::Mocks.space || !::RSpec::Mocks.space.registered?(object) || NilClass === object
+        dump_without_mocks(object, *rest)
+      else
+        dump_without_mocks(object.dup, *rest)
       end
-
-      dump_without_mocks(*args.unshift(object.dup))
     end
 
     alias_method :dump_without_mocks, :dump

spec/rspec/mocks/extensions/marshal_spec.rb

@@ -0,0 +1,48 @@
+require 'spec_helper'
+
+describe Marshal, 'extensions' do
+  # An object that raises when code attempts to dup it.
+  #
+  # Because we manipulate the internals of RSpec::Mocks.space below, we need
+  # an object that simply blows up when #dup is called without using any
+  # partial mocking or stubbing from rspec-mocks itself.
+  class UndupableObject
+    def dup
+      raise NotImplementedError
+    end
+  end
+
+  describe '#dump' do
+    context 'when rspec-mocks has not been fully initialized' do
+      def without_space
+        stashed_space, RSpec::Mocks.space = RSpec::Mocks.space, nil
+        yield
+      ensure
+        RSpec::Mocks.space = stashed_space
+      end
+
+      it 'does not duplicate the object before serialization' do
+        obj = UndupableObject.new
+        without_space do
+          expect { Marshal.dump(obj) }.not_to raise_error
+        end
+      end
+    end
+
+    context 'when rspec-mocks has been fully initialized' do
+      it 'duplicates objects with stubbed or mocked implementations before serialization' do
+        obj = double(:foo => "bar")
+        expect { Marshal.dump(obj) }.not_to raise_error
+      end
+
+      it 'does not duplicate other objects before serialization' do
+        obj = UndupableObject.new
+        expect { Marshal.dump(obj) }.not_to raise_error
+      end
+
+      it 'does not duplicate nil before serialization' do
+        expect { Marshal.dump(nil) }.not_to raise_error
+      end
+    end
+  end
+end