Use a frozen Set instance for CGI_VARIABLES.

This pull request switches the ActionDispatch::Http::Headers class from using a standard Array instance to a frozen Set instance for the CGI_VARIABLE constant. Since the constant usage is solely for internal list contents querying and should never contain non-unique values or be modifiable at runtime, a Set is more appropriate here, seemingly.

I've also expanded the header name list to be alphabetized across multiple lines for cleaner diffs and legibility in the future.