Update the type conversion to according to the types that mysql driver
returns: https://github.com/go-sql-driver/mysql/blob/v1.9.3/fields.go
All `scanTypeBytes` or `scanTypeString` or `scanTypeNullString` will now
be converted into string. Remaining types (numeric types or
`scanTypeUnknown`) are all returned as is.
Separately handle "JSON" to prevent double marshaling.
Fixes#840
