Fix xsd attributes in Firefox

[adube]

Fixes #1633.

Live example:

• https://adube.github.io/ngeo/fix-xsdattributes-ff/examples/attributes.html

[pgiraud]

@fredj What do you think about this? Have you ever heard of problems with FF not reading a document propertly if not given the correct namespace?

[fredj]

And what about always using the namespace?

[gberaudo]

And what about always using the namespace?

Is there a requirement that the namespace prefix be xsd?

[adube]

And what about always using the namespace?

When using the namespace, it works in FF, but not in Chrome.