refactor: uses new casing in header overrides

Author: JasonTheAdams

src/Providers/Http/Collections/HeadersCollection.php

@@ -115,15 +115,17 @@ private function set(string $name, $value): void
         }
         $lowerName = strtolower($name);
 
-        // If header exists with different casing, use the existing casing
+        // If header exists with different casing, remove the old casing
         if (isset($this->headersMap[$lowerName])) {
-            $actualName = $this->headersMap[$lowerName];
-            $this->headers[$actualName] = $normalizedValues;
-        } else {
-            // New header, use provided casing
-            $this->headers[$name] = $normalizedValues;
-            $this->headersMap[$lowerName] = $name;
+            $oldName = $this->headersMap[$lowerName];
+            if ($oldName !== $name) {
+                unset($this->headers[$oldName]);
+            }
         }
+
+        // Always use the new casing
+        $this->headers[$name] = $normalizedValues;
+        $this->headersMap[$lowerName] = $name;
     }
 
     /**

tests/unit/Providers/Http/Collections/HeadersCollectionTest.php

@@ -83,15 +83,16 @@ public function testCaseInsensitiveReplacement(): void
     {
         $headers = new HeadersCollection(['Content-Type' => 'application/json']);
 
-        // withHeader with different casing should replace existing
+        // withHeader with different casing should replace existing with new casing
         $newHeaders = $headers->withHeader('content-type', 'text/html');
 
         $this->assertEquals(['text/html'], $newHeaders->get('Content-Type'));
+        $this->assertEquals(['text/html'], $newHeaders->get('content-type'));
 
-        // Original casing should be preserved
+        // New casing should be used
         $all = $newHeaders->getAll();
-        $this->assertArrayHasKey('Content-Type', $all);
-        $this->assertArrayNotHasKey('content-type', $all);
+        $this->assertArrayHasKey('content-type', $all);
+        $this->assertArrayNotHasKey('Content-Type', $all);
     }
 
     /**