Support passing a callable to EnumValidator

In case the possible values are dynamic. This probably can't ever be
made to work reliably with statically generating HTML documentation.

It's expected that the proc never throws an exception.

lib/apipie/validator.rb

@@ -177,23 +177,23 @@ def description
     class EnumValidator < BaseValidator
       def initialize(param_description, argument)
         super(param_description)
-        @array = argument
+        @argument = argument
       end
 
       def validate(value)
-        @array.include?(value)
+        values.include?(value)
       end
 
       def self.build(param_description, argument, options, proc)
-        self.new(param_description, argument) if argument.is_a?(Array)
+        self.new(param_description, argument) if argument.is_a?(Array) || argument.respond_to?(:call)
       end
 
       def values
-        @array
+        @argument.respond_to?(:call) ? @argument.call : @argument
       end
 
       def description
-        string = @array.map { |value| format_description_value(value) }.join(', ')
+        string = values.map { |value| format_description_value(value) }.join(', ')
         "Must be one of: #{string}."
       end
     end

spec/lib/apipie/validator_spec.rb

@@ -133,17 +133,34 @@
   end
 
   describe 'EnumValidator' do
-    it "validates by object class" do
-      validator = Apipie::Validator::EnumValidator.new(params_desc, ['first', 'second & third'])
-      expect(validator.validate("first")).to be_truthy
-      expect(validator.validate("second & third")).to be_truthy
-      expect(validator.validate(1)).to be_falsey
-      expect(validator.validate({ 1 => 1 })).to be_falsey
+    context 'with an array' do
+      subject(:validator) { Apipie::Validator::EnumValidator.new(params_desc, ['first', 'second & third']) }
+
+      it "validates by object class" do
+        expect(validator.validate("first")).to be_truthy
+        expect(validator.validate("second & third")).to be_truthy
+        expect(validator.validate(1)).to be_falsey
+        expect(validator.validate({ 1 => 1 })).to be_falsey
+      end
+
+      it "has a valid description" do
+        expect(validator.description).to eq('Must be one of: <code>first</code>, <code>second &amp; third</code>.')
+      end
     end
 
-    it "has a valid description" do
-      validator = Apipie::Validator::EnumValidator.new(params_desc, ['first', 'second & third'])
-      expect(validator.description).to eq('Must be one of: <code>first</code>, <code>second &amp; third</code>.')
+    context 'with a callable' do
+      subject(:validator) { Apipie::Validator::EnumValidator.new(params_desc, -> { ['first', 'second & third'] } ) }
+
+      it "validates by object class" do
+        expect(validator.validate("first")).to be_truthy
+        expect(validator.validate("second & third")).to be_truthy
+        expect(validator.validate(1)).to be_falsey
+        expect(validator.validate({ 1 => 1 })).to be_falsey
+      end
+
+      it "has a valid description" do
+        expect(validator.description).to eq('Must be one of: <code>first</code>, <code>second &amp; third</code>.')
+      end
     end
   end
 end