Fix: Check envs for true/false

Fixes #1059.

The environments passed to createEnvironmentConfig were not being checked for true/false - thus they were being merged in even if the user config disabled them.
This test and the others like it seem to be a little brittle and opaque. This test was actually "failing" - that is, it was verifying the wrong length of output if both the browser and node environments had not been enabled. However it's very hard to tell unless you copy the command line and run it yourself.