Fix report request crash when using dup. attr idents

When request to /report endpoint with multiple attributes using the same
ident was used, request crashed with an error that Scan expects N+M
destinations but got N.

This was due to the way scan destination slice was constructed, using
column ident as unique key. Only problem for aggregations.

Example:
/report?metrics=SUM(Amount)+AS+sum_amount,SUM(Amount)+AS+sum_amount

Error:
sql: expected 4 destination arguments in Scan, not 3"

This is now done only for attributs with store type SimpleJsonDocColumn.

Ref: #465

store/adapters/rdbms/drivers/table.go

@@ -54,12 +54,12 @@ func NewTableCodec(m *dal.Model, d Dialect) *GenericTableCodec {
 		attr = m.Attributes[a]
 		colIdent = attr.StoreIdent()
 
-		if done[colIdent] {
-			continue
-		}
-
 		switch attr.Store.(type) {
 		case *dal.CodecRecordValueSetJSON:
+			if done[colIdent] {
+				continue
+			}
+
 			// when dealing with encoded types there is probably
 			// a different column that can handle the encoded payload
 			cols = append(cols, &SimpleJsonDocColumn{