fix: explodes set header strings

Author: JasonTheAdams

src/Providers/Http/Collections/HeadersCollection.php

@@ -107,7 +107,12 @@ public function has(string $name): bool
      */
     private function set(string $name, $value): void
     {
-        $normalizedValues = is_array($value) ? array_values($value) : [$value];
+        if (is_array($value)) {
+            $normalizedValues = array_values($value);
+        } else {
+            // Split comma-separated string into array
+            $normalizedValues = array_map('trim', explode(',', $value));
+        }
         $lowerName = strtolower($name);
 
         // If header exists with different casing, use the existing casing

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

@@ -30,6 +30,22 @@ public function testConstructorWithHeaders(): void
         $this->assertEquals(['value1', 'value2'], $headers->get('X-Custom'));
     }
 
+    /**
+     * Tests constructor with comma-separated string values.
+     *
+     * @return void
+     */
+    public function testConstructorWithCommaSeparatedString(): void
+    {
+        $headers = new HeadersCollection([
+            'Accept' => 'application/json, text/html, application/xml',
+            'Cache-Control' => 'no-cache, no-store',
+        ]);
+
+        $this->assertEquals(['application/json', 'text/html', 'application/xml'], $headers->get('Accept'));
+        $this->assertEquals(['no-cache', 'no-store'], $headers->get('Cache-Control'));
+    }
+
     /**
      * Tests case-insensitive header access.
      *
@@ -126,4 +142,17 @@ public function testWithHeaderImmutability(): void
         $this->assertEquals(['text/html'], $new->get('Accept'));
         $this->assertEquals(['application/json'], $original->get('Content-Type'));
     }
+
+    /**
+     * Tests withHeader with comma-separated string.
+     *
+     * @return void
+     */
+    public function testWithHeaderCommaSeparatedString(): void
+    {
+        $headers = new HeadersCollection();
+        $new = $headers->withHeader('Accept', 'application/json, text/html, application/xml');
+
+        $this->assertEquals(['application/json', 'text/html', 'application/xml'], $new->get('Accept'));
+    }
 }