Export: Fix bug with ID property search

Export: Fix bug with ID property search

Jan Böcker writes:

> If you have a headline with an elisp code block containing the following
> line:
>
> " :ID:"
>
> the HTML code will be garbled at the beginning of the headline.
>
> I have attached a minimal test case and the resulting HTML file. The
> #+OPTIONS: line is not needed, but is included to make the HTML file
> less cluttered.
>
> There has to be whitespace between the " and :ID: and the string must be
> ended on the same line. For example, these lines trigger the bug:
>
> " :ID:"
> "   :ID:"
> " :ID: garble-my-html"
>
> while these do not:
>
> ":ID:"
> ":ID: garble-my-html"
> " :ID:
>