(#21427) make supported_formats return an array of symbols

Prior to this commit, supported_formats was returning an array of
strings, which violated the contract for FormatHandler's
most_suitable_format_for method.  This commit fixes supported_formats
to return an array of symbols.
Merge pull request #1874 from kylog/issue/21427-fixup

(#21427) make supported_formats return an array of symbols