Land rapid7#20346, fixes payload encoding and substitutes for smaller base64 encoder

Use the smaller base64 encoder

Author: msutovsky-r7

modules/exploits/multi/http/php_fpm_rce.rb

@@ -51,6 +51,7 @@ def initialize(info = {})
               'Platform' => 'php',
               'Arch' => ARCH_PHP,
               'Payload' => {
+                'Encoder' => 'php/base64',
                 'PrependEncoder' => 'php -r "',
                 'AppendEncoder' => '"'
               }
@@ -310,7 +311,7 @@ def send_payload
       allow_retry: false
     )
     Rex.sleep(1)
-    return session_created? ? true : nil
+    return session_created? || nil
   end
 
   def send_backdoor_cleanup