Add support for except

In grape a new field was added for the values matcher called except in which you can pass in values that are not allowed to be entered for submission which can be found here. https://github.com/ruby-grape/grape/pull/1486
Currently through a normal curl call this works as expected but on the swagger side you can see here http://prntscr.com/ct2ime that the field is showing up as a blank drop-down.
How we are currently using it is :
requires :field, allow_blank: false, values: { except: -> { Model.all.map { |md| md&.assoc&.value } } , except_message: 'has a value that already exists' }

It would probably be nice to have in grape-swagger if the only thing that's passed through is 'except' the field generated should show up as a textbox instead of a drop down?

Reproduce the issue in grape-swagger using the shown requires declaration with values: { except: ... }, then compare the normal curl behavior with the generated Swagger field. Trace how the values matcher is represented in the Swagger output; done means an except-only field is presented as a textbox rather than a blank dropdown.