fix(qb): Fixed group by generation

* In qb/select.go, if there are no colums and the query have a GroupBy
statement then the query will be malformated.
* In qb/utils.go, refactor writeCql with a strings.Join call.

qb/utils.go

@@ -6,6 +6,7 @@ package qb
 
 import (
 	"bytes"
+	"strings"
 )
 
 // placeholders returns a string with count ? placeholders joined with commas.
@@ -24,10 +25,5 @@ func placeholders(cql *bytes.Buffer, count int) {
 type columns []string
 
 func (cols columns) writeCql(cql *bytes.Buffer) {
-	for i, c := range cols {
-		cql.WriteString(c)
-		if i < len(cols)-1 {
-			cql.WriteByte(',')
-		}
-	}
+	cql.WriteString(strings.Join(cols, ","))
 }