Fix recent bug with timestamp properties

Fix recent bug with timestamp properties

Matt Lundin writes

> I believe that commit b8e0d6fdb41f2165d675e89fcb54b741c971f6f4 broke
> accessing timestamps with the org-entry-get.
>
> With that commit, several functions I use to check whether an entry has
> a timestamp stopped working.
>
> In other words,
>
> (org-entry-get nil "TIMESTAMP_IA")
>
> or
>
> (org-entry-get nil "TIMESTAMP")
>
> always return nil, even if a timestamp exists.
>
> Strangely, the org-entry-properties alist includes values for TIMESTAMP
> and TIMESTAMP_IA.
>
> I tested this by evaluating the expressions in the sample entry below:
>
> --8<---------------cut here---------------start------------->8---
> * TODO Test							   :computer:
>  <2009-12-19 Sat>
>  [2009-12-19 Sat 17:47]
>
> (org-entry-get nil "TIMESTAMP_IA")
> (org-entry-get nil "TIMESTAMP")
> (org-entry-properties)
> --8<---------------cut here---------------end--------------->8---