Use valid mediatype for GPX export.

This pull request fixes an issue in the GPX example using Chrome 37 where the button "export GPX" doesn't do anything.

Cause: in Chrome 37 a grammar check on the mediatype part in a data URI was introduced.
https://chromium.googlesource.com/chromium/src/+/5a665643fafcb4f102c4fb26f64e586496f46600