Fix github issue 267, GPX: handling of desc attribute.

Fixes: https://github.com/openlayers/openlayers/issues/267

Original description of the issue:

> In a GPX file, the description element is called 'desc', and the Format read creates a feature attribute 'desc'. However, write() looks for feature.attributes.description, which of course doesn't exist. The tests are also converting feature.attributes.description to desc, so it seems this is deliberate. Is this to make it compatible with KML? If so, it would surely be better to look for feature.attributes.desc and feature.attributes.description.

This fix includes a roundtrip test: an xml \<desc> is properly read into an attribute .desc, and then properly written into a \<desc> node, and then read into an attribute .desc again.

A second test checks that an explicity written .description attribute will be properly written to a \<desc> node, as was the previous behaviour, in order to avoid breaking existing code.

I am the author/copyright holder of this code and am dedicating it to the public domain under the terms at http://creativecommons.org/publicdomain/zero/1.0/.