Feature - add compression support (#1)

* Added option for compress request payload
* Add compression into $defaultOptions

---------

Co-authored-by: Sergіy Yegorchev <s.yegorchev@svitla.com>
Co-authored-by: Ricardo Branco <ricardo.branco@xtremepush.com>

Author: 3 people

lib/SparkPost/SparkPost.php

@@ -41,7 +41,8 @@ class SparkPost
         'version' => 'v1',
         'async' => true,
         'debug' => false,
-        'retries' => 0
+        'retries' => 0,
+        'compression' => false,
     ];
 
     /**
@@ -186,6 +187,12 @@ public function buildRequestValues($method, $uri, $payload, $headers)
 
         // old form-feed workaround now removed
         $body = json_encode($body);
+
+        if (!empty($this->options['compression']) && $this->options['compression'] === true) {
+            $headers['Content-Encoding'] = 'gzip';
+            $body = gzencode($body);
+        }
+
         return [
             'method' => $method,
             'url' => $url,