Expand test cases to cover current Docker handling

Examination of docker's handling showed a few additional cases were
needed.

tests/unit/test_env.py

@@ -11,26 +11,39 @@ from getpass import getuser
 def test_env():
     """
     Validate that you can define environment variables
+
+    See https://gist.github.com/hwine/9f5b02c894427324fafcf12f772b27b7
+    for how docker handles its -e & --env argument values
     """
     ts = str(time.time())
     with tempfile.TemporaryDirectory() as tmpdir:
         username = getuser()
         os.environ["SPAM"] = "eggs"
+        os.environ["SPAM_2"] = "ham"
         result = subprocess.run(
             [
                 "repo2docker",
+                # 'key=value' are exported as is in docker
                 "-e",
                 "FOO={}".format(ts),
                 "--env",
                 "BAR=baz",
+                # 'key' is exported with the currently exported value
                 "--env",
                 "SPAM",
+                # 'key' is not exported if it is not exported.
+                "-e",
+                "NO_SPAM",
+                # 'key=' is exported in docker with an empty string as
+                # value
+                "--env",
+                "SPAM_2=",
                 "--",
                 tmpdir,
                 "/bin/bash",
                 "-c",
                 # Docker exports all passed env variables, so we can
-                # just look at that output
+                # just look at exported variables.
                 "export",
             ],
             universal_newlines=True,
@@ -38,9 +51,12 @@ def test_env():
             stderr=subprocess.PIPE,
         )
     assert result.returncode == 0
+
     # all docker output is returned by repo2docker on stderr
     # extract just the declare for better failure message formatting
     declares = [x for x in result.stderr.split("\n") if x.startswith("declare")]
     assert 'declare -x FOO="{}"'.format(ts) in declares
     assert 'declare -x BAR="baz"' in declares
     assert 'declare -x SPAM="eggs"' in declares
+    assert "declare -x NO_SPAM" not in declares
+    assert 'declare -x SPAM_2=""' in declares