minor #7940 [Form] Fix choice keys and values for custom field types (michaelperrin)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7940).

Discussion
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[Form] 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).

I prefered to keep the same parameters for the "Creating your Field Type as a Service" section and call `array_flip` as I think it is clearer to keep the keys as keys (!) for parameters, especially as the labels could contain some spaces.

Commits
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74b662b Fix choice keys and values for custom field types

Author: xabbuh

form/create_custom_field_type.rst

@@ -32,9 +32,9 @@ for form fields, which is ``<BundleName>\Form\Type``. Make sure the field extend
         {
             $resolver->setDefaults(array(
                 'choices' => array(
-                    'standard' => 'Standard Shipping',
-                    'expedited' => 'Expedited Shipping',
-                    'priority' => 'Priority Shipping',
+                    'Standard Shipping' => 'standard',
+                    'Expedited Shipping' => 'expedited',
+                    'Priority Shipping' => 'priority',
                 )
             ));
         }
@@ -394,7 +394,7 @@ method to ``ShippingType``, which receives the shipping configuration::
         public function configureOptions(OptionsResolver $resolver)
         {
             $resolver->setDefaults(array(
-                'choices' => $this->shippingOptions,
+                'choices' => array_flip($this->shippingOptions),
             ));
         }