Multi encode issue 113

Closes issue #113 without breaking changes to previous tests.

This patch prevents nasty double encoding issue they happens when saving an attachment after initial upload. As detailed below:
```
> Attachment.last.save!
   (0.4ms)  UPDATE "attachments" SET "asset" = '6a64205d-ab7e-45bf-97bb-5aee15491fe5/Pre-review%2520Evaluation.pdf', "updated_at" = '2014-04-01 01:02:11.677369' WHERE "attachments"."id" = 194

> Attachment.last.save!
   (0.3ms)  UPDATE "attachments" SET "asset" = '6a64205d-ab7e-45bf-97bb-5aee15491fe5/Pre-review%25252520Evaluation.pdf', "updated_at" = '2014-04-01 01:02:35.526486' WHERE "attachments"."id" = 194

> Attachment.last.save!
   (0.3ms)  UPDATE "attachments" SET "asset" = '6a64205d-ab7e-45bf-97bb-5aee15491fe5/Pre-review%252525252520Evaluation.pdf', "updated_at" = '2014-04-01 01:02:45.493389' WHERE "attachments"."id" = 194
```

Specs added to prove prevention of double encoding.