[processor] Prevent accidental calls to global functions

if any of the values is the string 'header' or 'die'
then that global function gets called.
This should be done explicitly if indended.

Issue: MOYA-1244

src/Processor/CallableContextProcessor.php

@@ -27,7 +27,7 @@ public function __invoke(array $record)
 		}
 
 		foreach ($record['context'] as $key => &$value) {
-			if (\is_callable($value)) {
+			if (!is_string($value) && \is_callable($value)) {
 				try {
 					$value = $value();
 				}