livecheck: restrict POST hashes to symbol keys

[samford]

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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.