Prevent cURL transport from leaking on Exception

The `CURLOPT_HEADERFUNCTION` and `CURLOPT_WRITEFUNCTION` options are not
being reset if a cURL handle error occurs (which throws a
`Requests_Exception` and stops execution). The destructor is not called
due to lingering instance method callbacks in the two options.

Move `CURLOPT_HEADERFUNCTION` and `CURLOPT_WRITEFUNCTION` cleanup before
any exceptions can happen.

src/Transport/Curl.php

@@ -200,12 +200,14 @@ public function request($url, $headers = array(), $data = array(), $options = ar
 			$response = $this->response_data;
 		}
 
-		$this->process_response($response, $options);
+		if (true === $options['blocking']) {
+			// Need to remove the $this reference from the curl handle.
+			// Otherwise \WpOrg\Requests\Transport\Curl wont be garbage collected and the curl_close() will never be called.
+			curl_setopt($this->handle, CURLOPT_HEADERFUNCTION, null);
+			curl_setopt($this->handle, CURLOPT_WRITEFUNCTION, null);
+		}
 
-		// Need to remove the $this reference from the curl handle.
-		// Otherwise \WpOrg\Requests\Transport\Curl won't be garbage collected and the curl_close() will never be called.
-		curl_setopt($this->handle, CURLOPT_HEADERFUNCTION, null);
-		curl_setopt($this->handle, CURLOPT_WRITEFUNCTION, null);
+		$this->process_response($response, $options);
 
 		return $this->headers;
 	}