implemented except in values validator

[jonmchan]

This implements the feature requested in #1485 .

Some notes:

• This PR allows for except to be used exclusively allowing all other values except what is defined in except as well as with values, defining a range and then marking a subset of the range as invalid.
• This PR allows for custom except message outside of the regular validation fail message, but falls back to the regular validation fail message if no except_message is defined.
• There was slight difficulty maintaining compatibility with allowing values to be passed in to values: without the value: key. Please ensure that line 5-8 of values.rb https://github.com/ruby-grape/grape/compare/master...jonmchan:feature/except?expand=1#diff-45b62b4ed0aed7ceff346c347f76570eR8 do not cause any incompatibility issues.
• Side effect of this implementation is that values: [] accepts all passed parameters. Previous implementation, values: [] would reject every value. I figure nobody would be trying to create a parameter that would allow no value so I did not bother to maintain this functionality.

[jonmchan]

I don't know how to fix the danger error:

[!] Invalid `temporary_danger.rb` file: undefined local variable or method `changelog' for #<Danger::Dangerfile:0x0000000378e850>. Updating the Danger gem might fix the issue.

I can't even run danger on my local system:

$ bundle exec danger
bundler: failed to load command: danger (/home/jchan/.rbenv/versions/2.3.1/bin/danger)
NameError: undefined local variable or method `ci_klass' for #<Danger::Executor:0x0056436b27bbe0>
Did you mean?  class
  /home/jchan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/danger-2.1.6/lib/danger/danger_core/executor.rb:16:in `run'
  /home/jchan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/danger-2.1.6/lib/danger/commands/runner.rb:56:in `run'
  /home/jchan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/claide-1.0.0/lib/claide/command.rb:334:in `run'
  /home/jchan/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/danger-2.1.6/bin/danger:5:in `<top (required)>'
  /home/jchan/.rbenv/versions/2.3.1/bin/danger:23:in `load'
  /home/jchan/.rbenv/versions/2.3.1/bin/danger:23:in `<top (required)>'

[jonmchan]

@dblock any feedback?

[dblock]

The danger problem fixed in #1480. Rebase.

[dblock]

Overall this looks great. My biggest question is whether it should be

requires :number, type: Integer, values: { value: 1..20 except: [4,13], except_message: 'includes unsafe numbers', message: 'is outside the range of numbers allowed' }

vs.

requires :number, type: Integer, values: 1..20, except: [4,13], except_message: 'includes unsafe numbers', message: 'is outside the range of numbers allowed'

The latter would be consistent with how we do all options today.

[jonmchan]

If I am understanding the difference in what you are suggesting, you are saying that:

#1.) value is omitted and #2.) the parameters are not passed into a hash to values.

However, I don't find this consistent with the currently implementation in terms of tests:

eg,

https://github.com/ruby-grape/grape/blob/master/spec/grape/validations/validators/values_spec.rb#L26

requires :type, values: { value: ValuesModel.values, message: 'value does not include in values' }

https://github.com/ruby-grape/grape/blob/master/spec/grape/validations/validators/values_spec.rb#L33

            optional :type, values: { value: -> { ValuesModel.values }, message: 'value does not include in values' }, default: 'valid-type2'

If you are interested in seeing values refactored, I can make that change, but the change you are asking is very different (and potentially breaking) to how it currently behaves.

I maintain that the current PR is more consistent to how values validator is currently implemented.

[jonmchan]

@dblock fixed all errors. This should be good to go unless you want me to refactor values.

[dblock]

Yes you're totally right @jonmchan. Merging this, looks great.