[public-api] bug fix: setting an attachment's body to null should delete the attachment

I updated the API_Attachments unit test in APITests.m so it works with the public-api branch.  While doing this, I discovered that setting an attachment body to null in updateBody:contentType:error: does not actually delete the attachment.  The problem was that TDAttachment.m was passing in a non-null TDBlobStoreWriter as the body parameter of TD_Database updateAttachment:body:type:..., which was expecting a null body to indicate that the attachment should be removed.  This pull requests contains the updated test and a fix in TDAttachment.m for the issue.