Message constructor accepts $headers values as string or string[]

src/Message/Message.php

@@ -22,13 +22,13 @@ abstract class Message implements MessageInterface
      * Create a new Message, optionally with headers and a body.
      *
      * $headers is an optional associative array with header field names as
-     * (string) keys and lists of header field values (string[]) as values.
+     * string keys and values as either string or string[].
      *
      * If no StreamInterface is provided for $body, the instance will create
      * a NullStream instance for the message body.
      *
      * @param array $headers Associative array with header field names as
-     *     (string) keys and lists of header field values (string[]) as values.
+     *     keys and values as string|string[]
      * @param StreamInterface|null $body A stream representation of the message
      *     entity body
      */
@@ -37,19 +37,17 @@ abstract class Message implements MessageInterface
         ?StreamInterface $body = null
     ) {
         $this->headers = new HeaderCollection();
-        if ($headers) {
+
         foreach ($headers as $name => $values) {
+            if (is_string($values)) {
+                $values = [$values];
+            }
             foreach ($values as $value) {
                 $this->headers[$name] = $value;
             }
         }
-        }
 
-        if ($body !== null) {
-            $this->body = $body;
-        } else {
-            $this->body = new Stream('');
-        }
+        $this->body = $body ?? new Stream('');
     }
 
     public function __clone()

src/Message/Request.php

@@ -34,7 +34,7 @@ class Request extends Message implements RequestInterface
      * Create a new Request.
      *
      * $headers is an optional associative array with header field names as
-     * (string) keys and lists of header field values (string[]) as values.
+     * string keys and values as either string or string[].
      *
      * If no StreamInterface is provided for $body, the instance will create
      * a NullStream instance for the message body.
@@ -42,7 +42,7 @@ class Request extends Message implements RequestInterface
      * @param string $method
      * @param string|UriInterface $uri
      * @param array $headers Associative array with header field names as
-     *     (string) keys and lists of header field values (string[]) as values.
+     *     keys and values as string|string[]
      * @param StreamInterface|null $body A stream representation of the message
      *     entity body
      */

tests/Message/MessageTest.php

@@ -7,7 +7,14 @@ use WellRESTed\Test\TestCase;
 
 class MessageTest extends TestCase
 {
-    public function testSetsHeadersOnConstruction(): void
+    public function testSetsHeadersWithStringValueOnConstruction(): void
+    {
+        $headers = ['X-foo' => 'bar'];
+        $message = new Response(200, $headers);
+        $this->assertEquals(['bar'], $message->getHeader('X-foo'));
+    }
+
+    public function testSetsHeadersWithArrayValueOnConstruction(): void
     {
         $headers = ['X-foo' => ['bar', 'baz']];
         $message = new Response(200, $headers);