Fix curl headers for solr > 5

library/Solarium/Core/Client/Adapter/Curl.php

@@ -156,7 +156,11 @@ class Curl extends Configurable implements AdapterInterface
         }
 
         if (!isset($options['headers']['Content-Type'])) {
-            $options['headers']['Content-Type'] = 'text/xml; charset=utf-8';
+            if($method == Request::METHOD_GET){
+                $options['headers']['Content-Type'] = 'application/x-www-form-urlencoded; charset=utf-8';
+            } else {
+                $options['headers']['Content-Type'] = 'application/xml; charset=utf-8';
+            }
         }
 
         // Try endpoint authentication first, fallback to request for backwards compatibility