should be able to pass nil to an optional array values param (#1559)

* fix - should be able to pass `nil` to an optional array values param

* update changelog

* oops

Author: ghiculescu

CHANGELOG.md

@@ -7,6 +7,7 @@
 
 #### Fixes
 
+* [#1559](https://github.com/ruby-grape/grape/pull/1559): You can once again pass `nil` to optional attributes with `values` validation set - [@ghiculescu](https://github.com/ghiculescu).
 * Your contribution here.
 
 ### 0.19.1 (1/9/2017)

lib/grape/validations/validators/values.rb

@@ -17,7 +17,7 @@ def validate_param!(attr_name, params)
         excepts = @excepts.is_a?(Proc) ? @excepts.call : @excepts
         param_array = params[attr_name].nil? ? [nil] : Array.wrap(params[attr_name])
 
-        if param_array.all? { |param| excepts.include?(param) }
+        if !param_array.empty? && param_array.all? { |param| excepts.include?(param) }
           raise Grape::Exceptions::Validation, params: [@scope.full_name(attr_name)], message: except_message
         end
 

spec/grape/validations/validators/values_spec.rb

@@ -159,6 +159,11 @@ class API < Grape::API
         get '/mixed/value/except' do
           { type: params[:type] }
         end
+
+        params do
+          optional :optional, type: Array[String], values: %w(a b c)
+        end
+        put '/optional_with_array_of_string_values'
       end
     end
   end
@@ -242,6 +247,11 @@ def app
       get('/optional_with_required_values')
       expect(last_response.status).to eq 200
     end
+
+    it 'allows for an optional param with a list of values' do
+      put('/optional_with_array_of_string_values', optional: nil)
+      expect(last_response.status).to eq 200
+    end
   end
 
   it 'allows a valid default value' do