Fix GET requests with bodies being sent as POST

While infrequently used, GET requests with bodies are allowed and used
by applications like elasticsearch. Fix GET requests being sent as post.

Fixes #13406

src/Http/Client/Adapter/Curl.php

@@ -96,6 +96,10 @@ class Curl implements AdapterInterface
         if ($body) {
             $body->rewind();
             $out[CURLOPT_POSTFIELDS] = $body->getContents();
+            // GET requests with bodies require custom request to be used.
+            if (isset($out[CURLOPT_HTTPGET])) {
+                $out[CURLOPT_CUSTOMREQUEST] = 'get';
+            }
         }
 
         if (empty($options['ssl_cafile'])) {

tests/TestCase/Http/Client/Adapter/CurlTest.php

@@ -119,6 +119,42 @@ class CurlTest extends TestCase
      *
      * @return void
      */
+    public function testBuildOptionsGetWithBody()
+    {
+        $options = [
+            'timeout' => 5,
+        ];
+        $request = new Request(
+            'http://localhost/things',
+            'GET',
+            ['Cookie' => 'testing=value'],
+            '{"some":"body"}'
+        );
+        $result = $this->curl->buildOptions($request, $options);
+        $expected = [
+            CURLOPT_URL => 'http://localhost/things',
+            CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
+            CURLOPT_RETURNTRANSFER => true,
+            CURLOPT_HEADER => true,
+            CURLOPT_HTTPHEADER => [
+                'Cookie: testing=value',
+                'Connection: close',
+                'User-Agent: CakePHP',
+            ],
+            CURLOPT_HTTPGET => true,
+            CURLOPT_POSTFIELDS => '{"some":"body"}',
+            CURLOPT_CUSTOMREQUEST => 'get',
+            CURLOPT_TIMEOUT => 5,
+            CURLOPT_CAINFO => $this->caFile,
+        ];
+        $this->assertSame($expected, $result);
+    }
+
+    /**
+     * Test converting client options into curl ones.
+     *
+     * @return void
+     */
     public function testBuildOptionsPost()
     {
         $options = [];