Bugfix for GELF-fields still being prefixed with _

Hi Amjad - I ran into two bugs with the Graylog2 extension for Zend Framework 1. One of them was resolved in a fork of your repo by @jbrinksmeier already, so I used his fork as a basis. Although I realize you haven't touched this code in two years, since your repo is the primary search result when people search for Zend Log + Graylog2, would you accept this pull request?