Manage "Expect" header with cURL transport

Fixes #453

library/Requests/Transport/cURL.php

@@ -317,6 +317,19 @@ protected function setup_handle($url, $headers, $data, $options) {
 			$headers['Connection'] = 'close';
 		}
 
+		// Add "Expect" header.
+		//
+		// By default, cURL adds a "Expect: 100-Continue" to most requests. This header can
+		// add as much as a second to the time it takes for cURL to perform a request. To
+		// prevent this, we need to set an empty "Expect" header. To match the behaviour of
+		// Guzzle, we'll add the empty header to requests that are smaller than 1 MB and use
+		// HTTP/1.1.
+		//
+		// https://curl.se/mail/lib-2017-07/0013.html
+		if (!isset($headers['Expect']) && 1.1 === $options['protocol_version']) {
+			$headers['Expect'] = strlen(is_array($arguments['body']) ? implode('', $body) : $arguments['body']) >= 1048576 ? '100-Continue' : '';
+		}
+
 		$headers = Requests::flatten($headers);
 
 		if (!empty($data)) {