IsSqlType instance is needed if the field is an array of enum

[stevemao]

when toFields is used on sql array, it uses this instance

(Default ToFields a (Field_ n b), IsSqlType b) => Default ToFields [a] (Field (SqlArray_ n b))

Which cast the value to the enum type

[tomjaguarpaw]

Thanks! At some point I'll fold this into EnumMapper.

[tomjaguarpaw]

I incorporated this into EnumMapper in https://github.com/tomjaguarpaw/haskell-opaleye/commit/91727ec6af166e6a5c9bdfa8cced4f41bcf9dc93