Disabling keep-alive for SOAP calls.

PHP SoapClient creates keep-alive connections but never reuse them.
Disabling this behavior. Should resolve issues #298 and #287.

src/Google/AdsApi/Common/AdsSoapClient.php

@@ -61,6 +61,10 @@ public function __construct($wsdl, array $options = null) {
     if (array_key_exists('classmap', $options)) {
       $this->classmap = $options['classmap'];
     }
+    // SoapClient sets keep alive header by default but does not re-use the
+    // connections. Disabling this to avoid running out of FD handles.
+    $options['keep_alive'] = false;
+
     $this->reflection = new Reflection();
     parent::__construct($wsdl, $options);
   }