Add gotcha to RDoc of collection_check_boxes

[maurogeorge]

The collection_check_boxes generates a default hidden input to prevent a gotcha, like the file_field does.

This PR add a RDoc to this.

[zzak]

Which spec?

Also, probably:

The HTML specification says that web browsers should not send any value when no selection is made for a collection of radio buttons.

or

When no selection is made for a collection of radio buttons most web browsers will not send any value, based on the HTML specification.

[maurogeorge]

@zzak thanks for the review.
I updated the doc following your observations.

[zzak]

Please wrap it at 80 chars, we may possibly want to consider the following:

From 8.1.2.4 from RFC1866

If none of the elements of a set of radio buttons specifies
`CHECKED', then the user agent must check the first radio button of
the set initially.

[maurogeorge]

@zzak I updated to wrap it on 80 chars.

Actually we are adding docs about collection of checkboxes and not radio buttons, yeah on my first commit this was sent as radio buttons, my mistake sorry 😞 .

As you can see on on 8.1.2.3 from RFC1866 we do not have the same doc of the radio buttons. So I rewrote the first paragraph.

This makes senses to you? Or you think we have some RFC that describes this behavior?

[zzak]

This looks good to me, thanks for your contribution @maurogeorge!

[maurogeorge]

Thanks @zzak for the review and merge