Utility\CaseInsensitiveDictionary: minor bug fix

The use of `strtolower()` on non-string keys, is 1) not necessary and 2) had side-effects when non-string keys were used as those were then cast to string, which with the current set of tests would lead to the `false` test case overwritting the `null` test case and potentially "passing null to non-nullable" errors in PHP 8.1.

While this class is intended to be used with requests headers, the class in itself is not limited to this use-case, so should function as per the specifications, independently of the use-case.

Alternatively, a stricter check could be put in place to only allow string keys for the data captured in this class. That would potentially create a bigger breaking change though.

src/Utility/CaseInsensitiveDictionary.php

@@ -47,7 +47,10 @@ public function __construct(array $data = array()) {
 	 */
 	#[ReturnTypeWillChange]
 	public function offsetExists($offset) {
-		$offset = strtolower($offset);
+		if (is_string($offset) {
+			$offset = strtolower($offset);
+		}
+
 		return isset($this->data[$offset]);
 	}
 
@@ -59,7 +62,10 @@ public function offsetExists($offset) {
 	 */
 	#[ReturnTypeWillChange]
 	public function offsetGet($offset) {
-		$offset = strtolower($offset);
+		if (is_string($offset)) {
+			$offset = strtolower($offset);
+		}
+
 		if (!isset($this->data[$offset])) {
 			return null;
 		}
@@ -81,7 +87,10 @@ public function offsetSet($offset, $value) {
 			throw new Exception('Object is a dictionary, not a list', 'invalidset');
 		}
 
-		$offset              = strtolower($offset);
+		if (is_string($offset)) {
+			$offset = strtolower($offset);
+		}
+
 		$this->data[$offset] = $value;
 	}
 
@@ -92,7 +101,11 @@ public function offsetSet($offset, $value) {
 	 */
 	#[ReturnTypeWillChange]
 	public function offsetUnset($offset) {
-		unset($this->data[strtolower($offset)]);
+		if (is_string($offset)) {
+			$offset = strtolower($offset);
+		}
+
+		unset($this->data[$offset]);
 	}
 
 	/**