strftime doesn't respect en-GB locale

[Pete Walker]

When you setup your locales, you assign the en-US the en locale by reference:

Date.ext.locales['en-US'] = Date.ext.locales.en;

You need to clone the object instead, as the changes you make are affecting the en-GB locale.
e.g.

Date.ext.locales['en-US'] = JSON.parse(JSON.stringify(Date.ext.locales.en));