Returns an query string for this up.Params instance.
The keys and values in the returned query string will be percent-encoded.
Non-primitive values (like File will be omitted from
the retuned query string.
Example
var params = new up.Params({ foo: 'bar', baz: 'bam' })
var query = params.toQuery()
// query is now: 'foo=bar&baz=bam'
