[Form] refactor CheckboxListMapper and RadioListMapper

fixes #14712 and #17789.

`ChoiceType` now always use `ChoiceToValueTransformer` or
`ChoicesToValuesTransformer` depending on `multiple` option.
Hence `CheckboxListMapper` and `RadioListMapper` don’t handle
the transformation anymore.
Fixes pre selection of choice with model values such as `null`,
`false` or empty string.

Author: HeahDude

Tests/Form/Type/EntityTypeTest.php

@@ -261,7 +261,7 @@ public function testSubmitSingleExpandedNull()
         $field->submit(null);
 
         $this->assertNull($field->getData());
-        $this->assertNull($field->getViewData());
+        $this->assertSame('', $field->getViewData(), 'View data is always a string');
     }
 
     public function testSubmitSingleNonExpandedNull()

composer.json

@@ -22,7 +22,7 @@
     "require-dev": {
         "symfony/stopwatch": "~2.2",
         "symfony/dependency-injection": "~2.2",
-        "symfony/form": "~2.7,>=2.7.1",
+        "symfony/form": "~2.7.12|~2.8.5",
         "symfony/http-kernel": "~2.2",
         "symfony/property-access": "~2.3",
         "symfony/security": "~2.2",