[O] Bug in HTML export

[Pavel Panchekha]

Ampersands in #+AUTHOR: declarations are not correctly escaped by Org-mode.

To reproduce, export:

#+AUTHOR: a & b

You'll find the line

<meta name="author" content="a & b"/>

in the output, but it should be

<meta name="author" content="a &amp; b"/>

The same problem occurs with other special characters in author declarations, and also in #+DATE: declarations.

This is breaking the "XHTML 1.0 Strict" claim.

- Pavel Panchekha