Fix choice keys and values for custom field types

Choices labels are defined as the keys of the array as of Symfony 3 (see symfony/symfony#14050).

Author: michaelperrin

form/create_custom_field_type.rst

@@ -33,8 +33,8 @@ for form fields, which is ``<BundleName>\Form\Type``. Make sure the field extend
         {
             $resolver->setDefaults(array(
                 'choices' => array(
-                    'm' => 'Male',
-                    'f' => 'Female',
+                    'Male' => 'm',
+                    'Female' => 'f',
                 )
             ));
         }
@@ -384,7 +384,7 @@ configuration::
         public function configureOptions(OptionsResolver $resolver)
         {
             $resolver->setDefaults(array(
-                'choices' => $this->genderChoices,
+                'choices' => array_flip($this->genderChoices),
             ));
         }