false defaults ignored/removed

Some spots where a default set to `false` are ignored/removed:
```clojure
user=> (cli/spec->opts {:flag {:default false} :flag2 {:default true}})
{:exec-args {:flag2 true}}

user=> (cli/format-opts {:spec {:flag {:default false} :flag2 {:default true}}})
"  --flag\n  --flag2 true"
```

Can provide a patch.