Test that should fail for #1437
Fix mangling of `wp user list`

1. Only JSON encode fields that will be displayed on output. `WP_User`'s `data` attribute was getting JSON-encoded, which meant the magic getter no longer worked.
2. Using `isset()` for object properties, instead of `property_exists()`, means we can check if magic properties are available.
Pre 5.5 backwards compat
Ensure mu-plugins conform to our established spec for plugins

Fixes this error: https://travis-ci.org/wp-cli/wp-cli/jobs/37127895
PHP 5.3 compat
Merge pull request #1437 from wp-cli/fix-1437

`wp user list` produces empty table