Remove prompt from boolean select field

[darinwilson]

This removes the "Choose one:" prompt when generating a boolean select field. This fixes issue #11.

If you supply a prompt option to phoenix_html's select function, it always adds the prompt as an option with an empty value. This doesn't really make sense for a boolean, which should only be true or false, never blank.

@yulianglukhenko correctly points out that placeholders for select fields should be disabled, rather than having a blank value. That forces the user to make a selection which, for an optional field, is not what you want. Ideally, select would also take a required option that would either force the user to select an option, or allow the blank value. That would make a nice PR for phoenix_html.

[yulolimum]

If this specific select is a boolean, wouldn't it be safe to just default to false? That would be the same as a nil for when you're doing logic, but would also pass the changeset validations on update.. @darinwilson

[zberkom]

Looks good!

[darinwilson]

@yulianglukhenko I'm not sure that false is necessarily any safer than true - it really depends on context. I'm inclined to err on the side of simplicity at this point and leave it as is. And since this is just a generator, the user can customize as they see fit.

[yulolimum]

Agreed - :)