Add unit test and update CHANGELOG

Author: tleonhardt

CHANGELOG.md

@@ -5,6 +5,7 @@
     * Renamed `install_command_set()` and `uninstall_command_set()` to `register_command_set()` and 
       `unregister_command_set()` for better name consistency.
 * Bug Fixes
+    * Fixed help formatting bug in `Cmd2ArgumentParser` when `nargs` > 1 and `metavar` is a tuple
     * Added explicit testing against python 3.5.2 for Ubuntu 16.04, and 3.5.3 for Debian 9
     * Added fallback definition of typing.Deque (taken from 3.5.4)
     * Removed explicit type hints that fail due to a bug in 3.5.2 favoring comment-based hints instead

tests/test_argparse_custom.py

@@ -260,3 +260,10 @@ def test_override_parser():
     # Verify DEFAULT_ARGUMENT_PARSER is now our CustomParser
     from examples.custom_parser import CustomParser
     assert DEFAULT_ARGUMENT_PARSER == CustomParser
+
+
+def test_apcustom_metavar_tuple():
+    # Test the case when a tuple metavar is used with nargs an integer > 1
+    parser = Cmd2ArgumentParser()
+    parser.add_argument('--aflag', nargs=2, metavar=('foo', 'bar'), help='This is a test')
+    assert '[--aflag foo bar]' in parser.format_help()