Throw an exception if the PHP value provided to DateTimeMapping is not a Carbon instance

Author: robgridley

src/Soap/DateTimeMapping.php

@@ -4,6 +4,7 @@
 
 use Carbon\Carbon;
 use SimpleXMLElement;
+use InvalidArgumentException;
 use Pace\Contracts\Soap\TypeMapping;
 
 class DateTimeMapping implements TypeMapping
@@ -55,6 +56,10 @@ public function fromXml($xml)
      */
     public function toXml($php)
     {
+        if (!$php instanceof Carbon) {
+            throw new InvalidArgumentException('PHP value must be a Carbon instance');
+        }
+
         return sprintf(
             '<%1$s>%2$s</%1$s>',
             $this->getTypeName(),