Improve type formatting logic in :cast function

[duddlf23]

In the :cast function (special syntax), if the type is keyword or symbol, it can be quoted according to the format option.

;; Example

(sql/format {:select [:foo [[:cast :bar :integer]]]} {:quoted true})
=> ["SELECT \"foo\", CAST(\"bar\" AS \"integer\")"]

(sql/format {:select [:foo [[:cast :bar :integer]]]} {:dialect :mysql})
=> ["SELECT `foo`, CAST(`bar` AS `integer`)"]

It is not valid that type to be quoted in the CAST( expr AS type ) syntax. Therefore, I modified the formatter of :cast to use sql-kw when the type is keyword or symbol. And I also added related tests.

[seancorfield]

Thank you, looks good.