Added default value for count

[benjaminwinger]

If count is used in a message, but not passed as a variable, it should provide a meaningful default.

I'm not really sure why you would want to implement a plural message without a count argument, but the tests didn't really make sense:

assert.equal("Une chose.", i18n('message', {count = 0})) is clearly incorrect, as while 0 is in the same CLDR plural category as 1 in French, it doesn't mean the same thing. So without the ability to match against numerical constants in addition to CLDR plural categories, it only works to have the message be:

i18n.set('fr.message', {
    one   = "%{count} chose.",
    other = "%{count} choses."
})

But then the defaults to 1 test produces nil chose., which isn't helpful, so we need a default value for count as well as a default plural category for it.