Change `symbolize_names` to `symbolize_keys`

This occured to me while testing this option without the docs in front of me; it didn't work because I thought the option was named symbolize_keys (and only one day before I wrote the docs for it...). Why symbolize_names? symbolize_keys describes exactly what it does, symbolizing Hash keys, not names. Also, symbolize_keys already has some spread as a method name.

I will provide a PR if there is agreement on this.

Start by searching the repository for symbolize_names, including its documentation and tests, then inspect how the option is parsed and exposed. Check whether symbolize_keys is already used elsewhere and define the compatibility scope; done means the agreed name is handled consistently and the relevant tests and docs pass.