[2.8] Allow empty lists as header values (#625)

Co-authored-by: Sean O'Brien <obrien.sean.dev@gmail.com>

Author: stobrien89

src/MessageTrait.php

@@ -174,10 +174,6 @@ private function normalizeHeaderValue($value): array
             return $this->trimAndValidateHeaderValues([$value]);
         }
 
-        if (count($value) === 0) {
-            throw new \InvalidArgumentException('Header value can not be an empty array.');
-        }
-
         return $this->trimAndValidateHeaderValues($value);
     }
 

tests/RequestTest.php

@@ -188,6 +188,14 @@ public function testHeaderValueWithWhitespace(): void
         ], $r->getHeaders());
     }
 
+    public function testEmptyListHeaderValue(): void
+    {
+        $r = new Request('GET', 'https://example.com/', ['Foo' => []]);
+        self::assertSame('', $r->getHeaderLine('Foo'));
+        self::assertSame([], $r->getHeader('Foo'));
+        self::assertSame(['Host' => ['example.com'], 'Foo' => []], $r->getHeaders());
+    }
+
     public function testCanGetHeaderAsCsv(): void
     {
         $r = new Request('GET', 'http://foo.com/baz?bar=bam', [

tests/ResponseTest.php

@@ -253,6 +253,14 @@ public function testPassNumericHeaderNameInConstructor(): void
         self::assertSame('bar', $r->getHeaderLine('123'));
     }
 
+    public function testConstructResponseEmptyListHeaderValue(): void
+    {
+        $r = new Response(200, ['Foo' => []]);
+        self::assertSame('', $r->getHeaderLine('Foo'));
+        self::assertSame([], $r->getHeader('Foo'));
+        self::assertSame(['Foo' => []], $r->getHeaders());
+    }
+
     /**
      * @dataProvider invalidHeaderProvider
      */
@@ -266,7 +274,6 @@ public function testConstructResponseInvalidHeader($header, $headerValue, $expec
     public static function invalidHeaderProvider(): iterable
     {
         return [
-            ['foo', [], 'Header value can not be an empty array.'],
             ['', '', '"" is not valid header name'],
             ['foo', new \stdClass(),  'Header value must be scalar or null but stdClass provided.'],
         ];