Bugfix for http://aosp/2319430 to parse env var

lexer_.ReadIdent() reads a single identifier, and cannot read the entire
environment value assignment - it stops at whitespace.

Also, it fails when attempting to unassign an environment variable.

This change also updates unit tests to test for these cases.

Change-Id: I9e4757f0323c9e095fbb7bac73954d6ca517f6d8
3 files changed