Fix bug with overriding #setup/#teardown on Minitest::Spec

If any of other test helping library calls ::MiniTest::Spec.before or
::MiniTest::Spec.after, the last one called wins. That should not be the case.
It makes for really had to track down bugs

Fixes mfpiccolo#24

lib/minitest-vcr/spec.rb

@@ -3,23 +3,24 @@
 
 module MinitestVcr
   module Spec
-
-    def self.configure!
-      run_before = lambda do |example|
+    module SetupAndTeardown
+      def setup
+        super
         if metadata[:vcr]
           options = metadata[:vcr].is_a?(Hash) ? metadata[:vcr] : {}
-          VCR.insert_cassette StringHelpers.vcr_path(example), options
+          VCR.insert_cassette StringHelpers.vcr_path(self), options
         end
       end
 
-      run_after = lambda do |example|
+      def teardown
+        super
         ::VCR.eject_cassette if metadata[:vcr]
       end
-
-      ::MiniTest::Spec.before :each, &run_before
-      ::MiniTest::Spec.after :each, &run_after
     end
 
+    def self.configure!
+      ::MiniTest::Spec.send(:include, SetupAndTeardown)
+    end
   end # Spec
 
   module StringHelpers

test/minitest-vcr/spec_test.rb

@@ -43,13 +43,9 @@
 
   describe "#configure!", :vcr do
 
-    before do
-      ::MiniTest::Spec.expects(:before).with(:each)
-      ::MiniTest::Spec.expects(:after).with(:each)
-    end
-
-    it "should call before and after with proper args and block" do
+    it "should include setup and teardown module into Minitest::Spec" do
       MinitestVcr::Spec.configure!
+      Minitest::Spec.included_modules.must_include(MinitestVcr::Spec::SetupAndTeardown)
     end
   end
 end