added support for guzzlehttp v2.6

Author: femz12

composer.json

@@ -8,7 +8,7 @@
     "require": {
         "php": ">=5.3.0",
         "psr/log": "^1.0 | ^2.0 | ^3.0",
-        "guzzlehttp/psr7": "^1.6",
+        "guzzlehttp/psr7": "^1.6 | ^2.6",
         "php-http/socket-client": "^1.4 | ^2.0",
         "php-http/message": "^1.7"
     },