fix(listen): don't panic with non-UTF8 environment
fix(listen): don't panic with non-UTF8 environment

Nickel::listen() and Nickel::listen_https() check for the presence
of $NICKEL_TEST_HARNESS in a way which causes a panic if any value
in the environment contains a non-UTF8 string. This patch fixes
the check.

Fixes #379

jolhoeft/nickel.rs,nickel-org/nickel.rs