I initially set the type for livecheck's `post_form` and `post_json`
hashes to allow either a string or symbol key. I used string keys in
the documentation, as there will inevitably be some form field names
that would pose a problem for symbols (e.g., `E-mail` uses a hyphen,
`1twothree` starts with a digit, etc.). However, I remembered that we
can simply use quote symbols like `:"E-mail"` to handle these
situations, as they have the flexibility of a string while still being
a symbol.
With that in mind, this updates related type signatures to only allow
symbol keys and updates documentation and tests accordingly. The
documentation example contains a hyphenated form field, so it
demonstrates how to handle names that don't work as a bare symbol.
5e57df7287