Do not leak TestUnit specific methods after Rails 4

* TestUnit requires the build_message method to be defined for its
  assertions to work properly. After Rails 4, Minitest is used instead
  of TestUnit so the method should not be included anymore. This is
  problematic because when using RSpec configured with

      config.expect_with :test_unit

  the TestUnit assertions try to call build_message on the
  MinitestAssertionAdapter which will crash.

Author: ferdynton

lib/rspec/rails/adapters.rb

@@ -201,7 +201,7 @@ def assertion_method_names
             select do |m|
               m.to_s =~ /^(assert|flunk|refute)/
             end
-          methods + [:build_message]
+          methods + test_unit_specific_methods
         end
 
         def define_assertion_delegators
@@ -211,6 +211,18 @@ def define_assertion_delegators
             end
           end
         end
+
+        # Starting on Rails 4, Minitest is the default testing framework so no
+        # need to add TestUnit specific methods.
+        if ::Rails::VERSION::STRING >= '4.0.0'
+          def test_unit_specific_methods
+            []
+          end
+        else
+          def test_unit_specific_methods
+            [:build_message]
+          end
+        end
       end
 
       class AssertionDelegator

spec/rspec/rails/assertion_adapter_spec.rb

@@ -25,4 +25,16 @@
   it "does not expose Minitest's message method" do
     expect(methods).not_to include("message")
   end
+
+  if ::Rails::VERSION::STRING >= '4.0.0'
+    # In Ruby <= 1.8.7 Object#methods returns Strings instead of Symbols. They
+    # are all converted to Symbols to ensure we always compare them correctly.
+    it 'does not leak TestUnit specific methods into the AssertionDelegator' do
+      expect(methods.map(&:to_sym)).to_not include(:build_message)
+    end
+  else
+    it 'includes methods required by TestUnit into the AssertionDelegator' do
+      expect(methods.map(&:to_sym)).to include(:build_message)
+    end
+  end
 end