Add tests and fix multi-value headers not being transformed.

I've also reworked how headers + server data are merged. The original
intent was to allow headers set in the request to overwrite server data,
which is still the case. These changes also allow headers not set in the
server environment to be transformed as well.

Refs #9967

src/Http/RequestTransformer.php

@@ -40,15 +40,12 @@ class RequestTransformer
     public static function toCake(PsrRequest $request)
     {
         $post = $request->getParsedBody();
-        $server = $request->getServerParams();
-        $headers = $request->getHeaders();
-
-        foreach ($headers as $k => $value) {
+        $headers = [];
+        foreach ($request->getHeaders() as $k => $value) {
             $name = sprintf('HTTP_%s', strtoupper(str_replace('-', '_', $k)));
-            if (isset($server[$name])) {
-                $server[$name] = $value[0];
-            }
+            $headers[$name] = implode(',', $value);
         }
+        $server = $headers + $request->getServerParams();
 
         $files = static::getFiles($request);
         if (!empty($files)) {

tests/TestCase/Http/RequestTransformerTest.php

@@ -87,12 +87,18 @@ class RequestTransformerTest extends TestCase
             'SERVER_PORT' => 443,
         ];
         $psr = ServerRequestFactory::fromGlobals($server);
+        $psr = $psr->withHeader('Api-Token', 'abc123')
+            ->withAddedHeader('X-thing', 'one')
+            ->withAddedHeader('X-thing', 'two');
+
         $cake = RequestTransformer::toCake($psr);
         $this->assertEmpty($cake->query);
         $this->assertEmpty($cake->data);
         $this->assertEmpty($cake->cookie);
 
         $this->assertSame('application/json', $cake->header('accept'));
+        $this->assertSame('abc123', $cake->header('Api-Token'));
+        $this->assertSame('one,two', $cake->header('X-thing'));
         $this->assertSame('PATCH', $cake->method());
         $this->assertSame('https', $cake->scheme());
         $this->assertSame(443, $cake->port());