Don't throw ClientException on 4XX

Some SRU servers return 4XX whenever there is a diagnostic message,
including zero results responses. We want to handle those responses by
our Response class.

src/Client.php

@@ -4,6 +4,7 @@
 use Http\Client\Common\PluginClient;
 use Http\Client\HttpClient;
 use Http\Discovery\HttpClientDiscovery;
+use Http\Client\Common\Exception\ServerErrorException;
 use Http\Client\Common\Plugin\ErrorPlugin;
 use Http\Discovery\MessageFactoryDiscovery;
 use Http\Message\Authentication\BasicAuth;
@@ -33,7 +34,7 @@ class Client
     protected $userAgent;
 
     /** @var array Custom headers */
-    protected $headers;
+    public $headers;
 
     /**
      * @var string|string[] Proxy configuration details.
@@ -66,7 +67,7 @@ public function __construct(
         $this->url = $url;
         $options = $options ?: array();
 
-        $plugins = [new ErrorPlugin()];
+        $plugins = [];
 
         $this->schema = isset($options['schema'])
             ? $options['schema']
@@ -214,6 +215,10 @@ public function request($method, $url)
         $request = $this->messageFactory->createRequest($method, $url, $this->headers);
         $response = $this->httpClient->sendRequest($request);
 
+        if ($response->getStatusCode() >= 500 && $response->getStatusCode() < 600) {
+            throw new ServerErrorException($response->getReasonPhrase(), $request, $response);
+        }
+
         return (string) $response->getBody();
     }
 }

tests/ClientTest.php

@@ -143,7 +143,7 @@ public function testHttpHeaders()
             'user-agent' => 'Blablabla/0.1',
         ));
 
-        $opts = $sru1->getHttpHeaders();
+        $opts = $sru1->headers;
 
         $this->assertEquals('application/xml', $opts['Accept']);
         $this->assertEquals('Blablabla/0.1', $opts['User-Agent']);