default headers will not override those set by user anymore

src/Client.php

@@ -117,11 +117,16 @@ public function doRequest($request)
             $this->httpClient->setUri($this->serverAddress);
         }
 
+        // set default Accept and Content-Type headers unless already set
         $headers = $httpRequest->getHeaders();
-        $headers->addHeaders([
-            'Content-Type' => 'application/json',
-            'Accept'       => 'application/json',
-        ]);
+        $headersToAdd = [];
+        if (!$headers->has('Content-Type')) {
+            $headersToAdd['Content-Type'] = 'application/json-rpc';
+        }
+        if (!$headers->has('Accept')) {
+            $headersToAdd['Accept'] = 'application/json-rpc';
+        }
+        $headers->addHeaders($headersToAdd);
 
         if (! $headers->get('User-Agent')) {
             $headers->addHeaderLine('User-Agent', 'Zend_Json_Server_Client');

test/ClientTest.php

@@ -14,6 +14,7 @@
 use Zend\Json\Server\Error;
 use Zend\Json\Server\Request;
 use Zend\Json\Server\Response;
+use Zend\Http\Response as HttpResponse;
 
 class ClientTest extends TestCase
 {
@@ -233,6 +234,41 @@ public function setServerResponseTo($nativeVars)
         $this->httpAdapter->setResponse($response);
     }
 
+    public function testClientShouldSetDefaultAcceptAndContentTypeHeadersOnRequest()
+    {
+        $request = new Request();
+        $response = new HttpResponse();
+        $response->setContent(\Zend\Json\Json::encode(['test' => 'test']));
+        $testAdapter = new TestAdapter();
+        $testAdapter->setResponse($response);
+        $jsonClient = new Client('http://foo');
+        $jsonClient->getHttpClient()->setAdapter($testAdapter);
+        $jsonClient->doRequest($request);
+        $this->assertSame('application/json-rpc', $jsonClient->getHttpClient()->getHeader('Content-Type'));
+        $this->assertSame('application/json-rpc', $jsonClient->getHttpClient()->getHeader('Accept'));
+    }
+
+    public function testClientShouldNotSetDefaultAcceptAndContentTypeHeadersOnRequestIfTheyAlreadyExist()
+    {
+        $request = new Request();
+        $response = new HttpResponse();
+        $response->setContent(\Zend\Json\Json::encode(['test' => 'test']));
+        $testAdapter = new TestAdapter();
+        $testAdapter->setResponse($response);
+
+        $httpClient = new HttpClient();
+        $httpClient->setHeaders([
+            'Content-Type' => 'application/jsonrequest',
+            'Accept' => 'application/jsonrequest',
+        ]);
+
+        $jsonClient = new Client('http://foo', $httpClient);
+        $jsonClient->getHttpClient()->setAdapter($testAdapter);
+        $jsonClient->doRequest($request);
+        $this->assertSame('application/jsonrequest', $jsonClient->getHttpClient()->getHeader('Content-Type'));
+        $this->assertSame('application/jsonrequest', $jsonClient->getHttpClient()->getHeader('Accept'));
+    }
+
     public function getServerResponseFor($nativeVars)
     {
         $response = new Response();