Changed empty function for choices field

Author: srtab

README.md

@@ -41,7 +41,7 @@ class ContactForm extends Form
     {
         // Use this function to crossfields validation
         //
-        // You can use $this->addError($message, $field) to add error messages to specific fields.
+        // You can use $this->addError($message, $field_name) to add error messages to specific fields.
         // Or just throw a ValidationError Exception.
     }
 }

src/Fields/ChoiceField.php

@@ -33,16 +33,21 @@ public function setChoices(array $choices)
         $this->widget->setChoices($choices);
     }
 
+    public function isEmpty($value)
+    {
+        return empty($value) && $value != "0";
+    }
+
     public function toNative($value)
     {
-        return !empty($value) ? (string) $value : '';
+        return !$this->isEmpty($value) ? (string) $value : '';
     }
 
     public function validate($value)
     {
         parent::validate($value);
 
-        if (empty($value) || !array_key_exists($value, $this->choices)) {
+        if ($this->isEmpty($value) || !array_key_exists($value, $this->choices)) {
             $message = Formatter::format($this->error_messages['invalid'], array(
                 'choice' => $value
             ));