Performs a server-side validation of a form field.
up.validate() submits the given field's form with an additional X-Up-Validate
HTTP header. Upon seeing this header, the server is expected to validate (but not save)
the form submission and render a new copy of the form with validation errors.
The unobtrusive variant of this is the input[up-validate] selector.
See the documentation for input[up-validate] for more information
on how server-side validation works in Unpoly.
Example
up.validate('input[name=email]', { target: '.email-errors' })
