Use a Glue test that doesn't destroy others

For some reason the test in this PR would make all other ones blow up.
For reasons not fully understood at the time of writing, the alternate
spec I'd written doesn't. So, we'll replace that test with this one and
it'll be green.

spec/paperclip/attachment_spec.rb

@@ -1490,25 +1490,4 @@ describe Paperclip::Attachment do
       assert_file_exists(@path)
     end
   end
-
-  context "Non ActiveRecord workflows" do
-    before do
-      # temporary take out AR
-      @ar_bak = Object.send(:remove_const, :ActiveRecord)
-    end
-
-    it "Can include glue without dependency on AR" do
-      klass = Object.const_set("SomeNonActiveRecordModel", Class.new)
-      klass.class_eval do
-        include Paperclip::Glue
-      end
-    end
-
-    after do
-      # put back AR
-      ActiveRecord = @ar_back
-    end
-  end
-
 end
-

spec/paperclip/glue_spec.rb

+# require "spec_helper"
+
+describe Paperclip::Glue do
+  describe "when ActiveRecord does not exist" do
+    before do
+      ActiveRecordSaved = ActiveRecord
+      Object.send :remove_const, "ActiveRecord"
+    end
+
+    after do
+      ActiveRecord = ActiveRecordSaved
+      Object.send :remove_const, "ActiveRecordSaved"
+    end
+
+    it "does not fail" do
+      NonActiveRecordModel = Class.new
+      NonActiveRecordModel.include(Paperclip::Glue)
+      Object.send :remove_const, "NonActiveRecordModel"
+    end
+  end
+
+  describe "when ActiveRecord does exist" do
+    before do
+      if Object.const_defined?("ActiveRecord")
+        @defined_active_record = false
+      else
+        ActiveRecord = :defined
+        @defined_active_record = true
+      end
+    end
+
+    after do
+      if @defined_active_record
+        Object.send :remove_const, "ActiveRecord"
+      end
+    end
+
+    it "does not fail" do
+      Foo = Class.new
+      Foo.include(Paperclip::Glue)
+      Object.send :remove_const, "Foo"
+    end
+  end
+end