Check predefined values for the request field and body part configuration option

See related issue https://github.com/smartanthill/smartanthill-zepto-compiler/issues/9

Need to handle this `_values` as `<select>` HTML element when:
- ... "Run" bodypart, where `_values` can be reflected for the `request` field.
- ... add new bodypart and option has this `_values` data.
