NODE_PATH was overridden by $_SERVER value

On BaseNodeFilter, NODE_PATH was overridden by $_SERVER value, even if it was set as an argument.

NODE_PATH can be set as the 2nd argument of a NodeFilter. The problem is that NODE_PATH is set, then $_SERVER environment is merged. If $_SERVER['NODE_PATH'] is set, it overrides previous value.

Also: $_SERVER was never merged with node environment unless NODE_PATH is set

I tried to make this change on 1.1.x branch, but I can't manage to make the test run. Travis build fail even on previous commit (before my change)