Fix environment merge from server var

[leblanc-simon]

Sometimes with PHP-FPM and Apache there is in the $_SERVER a key named "proxy-nokeepalive"
You can also define custom environment variables in your webserver and this variables will be passed to the global var $_SERVER (in Apache, try : SetEnv bad-name 1).

This environment variables could be named with a bad name for Linux/Windows environment variables.

In my webserver, when I use CompassFilter, the result is :
sh: 1: export: proxy-nokeepalive: bad variable name

The BaseNodeFilter has the same problem (use the BaseProcessFilter::mergeEnv method)

This fix check the var names before to add it in the environment of the process.