diff --git a/src/Http/Client/Adapter/Stream.php b/src/Http/Client/Adapter/Stream.php
index 1b2ddce0873..9619429c923 100644
--- a/src/Http/Client/Adapter/Stream.php
+++ b/src/Http/Client/Adapter/Stream.php
@@ -145,8 +145,8 @@ protected function _buildContext(Request $request, $options)
     protected function _buildHeaders(Request $request, $options)
     {
         $headers = [];
-        foreach ($request->headers() as $name => $value) {
-            $headers[] = "$name: $value";
+        foreach ($request->getHeaders() as $name => $values) {
+            $headers[] = sprintf('%s: %s', $name, implode(", ", $values));
         }
 
         $cookies = [];
@@ -171,23 +171,12 @@ protected function _buildHeaders(Request $request, $options)
      */
     protected function _buildContent(Request $request, $options)
     {
-        $content = $request->body();
-        if (empty($content)) {
-            return;
+        $body = $request->getBody();
+        if (empty($body)) {
+            return $this->_contextOptions['content'] = '';
         }
-        if (is_string($content)) {
-            $this->_contextOptions['content'] = $content;
-            return;
-        }
-        if (is_array($content)) {
-            $formData = new FormData();
-            $formData->addMany($content);
-            $type = $formData->contentType();
-            $request->header('Content-Type', $type);
-            $this->_contextOptions['content'] = (string)$formData;
-            return;
-        }
-        $this->_contextOptions['content'] = $content;
+        $body->rewind();
+        $this->_contextOptions['content'] = $body->getContents();
     }
 
     /**
diff --git a/src/Http/Client/Request.php b/src/Http/Client/Request.php
index 9edcd23ffda..398158b27f9 100644
--- a/src/Http/Client/Request.php
+++ b/src/Http/Client/Request.php
@@ -39,14 +39,10 @@ public function __construct()
     {
         $this->method = static::METHOD_GET;
 
-        $this->headerNames = [
-            'connection' => 'Connection',
-            'user-agent' => 'User-Agent',
-        ];
-        $this->headers = [
+        $this->header([
             'Connection' => 'close',
             'User-Agent' => 'CakePHP'
-        ];
+        ]);
     }
 
     /**
@@ -205,14 +201,13 @@ public function version($version = null)
     }
 
     /**
-     * Get/set the body for the message.
+     * Get/set the body/payload for the message.
      *
-     * *Warning* This method mutates the request in-place for backwards
-     * compatibility reasons, and is not part of the PSR7 interface.
+     * Array data will be serialized with Cake\Http\FormData,
+     * and the content-type will be set.
      *
      * @param string|null $body The body for the request. Leave null for get
      * @return mixed Either $this or the body value.
-     * @deprecated 3.3.0 use getBody() and withBody() instead.
      */
     public function body($body = null)
     {
@@ -220,6 +215,12 @@ public function body($body = null)
             $body = $this->getBody();
             return $body ? $body->__toString() : '';
         }
+        if (is_array($body)) {
+            $formData = new FormData();
+            $formData->addMany($body);
+            $this->header('Content-Type', $formData->contentType());
+            $body = (string)$formData;
+        }
         $stream = new Stream('php://memory', 'rw');
         $stream->write($body);
         $this->stream = $stream;
diff --git a/tests/TestCase/Network/Http/ClientTest.php b/tests/TestCase/Network/Http/ClientTest.php
index e2092dc2583..5a0568efb76 100644
--- a/tests/TestCase/Network/Http/ClientTest.php
+++ b/tests/TestCase/Network/Http/ClientTest.php
@@ -17,6 +17,7 @@
 use Cake\Network\Http\Request;
 use Cake\Network\Http\Response;
 use Cake\TestSuite\TestCase;
+use Zend\Diactoros\Uri;
 
 /**
  * HTTP client test.
@@ -369,7 +370,7 @@ public function testMethodsSimple($method)
             ->with($this->logicalAnd(
                 $this->isInstanceOf('Cake\Network\Http\Request'),
                 $this->attributeEqualTo('method', $method),
-                $this->attributeEqualTo('_url', 'http://cakephp.org/projects/add')
+                $this->attributeEqualTo('url', new Uri('http://cakephp.org/projects/add'))
             ))
             ->will($this->returnValue([$response]));
 
diff --git a/tests/TestCase/Network/Http/RequestTest.php b/tests/TestCase/Network/Http/RequestTest.php
index b4f44ac56a3..07b4e75f92c 100644
--- a/tests/TestCase/Network/Http/RequestTest.php
+++ b/tests/TestCase/Network/Http/RequestTest.php
@@ -111,6 +111,28 @@ public function testBody()
         $this->assertEquals($data, $request->body());
     }
 
+    /**
+     * test body method with array payload
+     *
+     * @return void
+     */
+    public function testBodyArray()
+    {
+        $request = new Request();
+        $data = [
+            'a' => 'b',
+            'c' => 'd',
+            'e' => ['f', 'g']
+        ];
+        $request->body($data);
+        $this->assertEquals('application/x-www-form-urlencoded', $request->getHeaderLine('content-type'));
+        $this->assertEquals(
+            'a=b&c=d&e%5B0%5D=f&e%5B1%5D=g',
+            $request->body(),
+            'Body should be serialized'
+        );
+    }
+
     /**
      * Test that body() modifies the PSR7 stream
      *