security issue with drop_invalid

[ngapaillard]

I found a security issue with the plugin. If the header "x-hub-signature" isn't in the request and drop_invalid is setted to "true" then the message isn't dropped...

• Version: latest

• Operating System: MACOS

• Config File :
  input {
  github {
  drop_invalid => true
  ip => "0.0.0.0"
  port => "8080"
  secret_token => "test"
  }
  }

• Steps to Reproduce:
  run
  curl -XPOST -d'test' localhost:8080
  The request won't be dropped by the plugin because the header "x-hub-signature" isn't present, so it pass the control...

[ngapaillard]

Invalid plugin, I will move to the good one