let gorm support limit and offset binding parameters,change the BindV…

…ar of postgres driver (#239)

* Update postgres.go

if limit and offset use the parameter  in new gorm version，must remove config vars before bindvar to ,and then restore them.

* optimise the BindVarTo code to ensure   the code  more efficient

* fix"for bug "  for bindvar

* remove cases

* remove for logic

* just check the 0 index

postgres.go

@@ -115,7 +115,15 @@ func (dialector Dialector) DefaultValueOf(field *schema.Field) clause.Expression
 
 func (dialector Dialector) BindVarTo(writer clause.Writer, stmt *gorm.Statement, v interface{}) {
 	writer.WriteByte('$')
-	writer.WriteString(strconv.Itoa(len(stmt.Vars)))
+	index := 0
+	varLen := len(stmt.Vars)
+	if varLen > 0 {
+		switch stmt.Vars[0].(type) {
+		case pgx.QueryExecMode:
+			index++
+		}
+	}
+	writer.WriteString(strconv.Itoa(varLen - index))
 }
 
 func (dialector Dialector) QuoteTo(writer clause.Writer, str string) {