fix : fail to alter existing boolean column from smallint with default value to boolean in postgres (go-gorm#180)

Author: jeffry-luqman

migrator.go

@@ -315,6 +315,11 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 						return err
 					}
 				} else {
+					if dv, _ := fieldColumnType.DefaultValue(); dv != "" {
+						if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? DROP DEFAULT", m.CurrentTable(stmt), clause.Column{Name: field.DBName}).Error; err != nil {
+							return err
+						}
+					}
 					if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? TYPE ?"+m.genUsingExpression(fileType.SQL, fieldColumnType.DatabaseTypeName()),
 						m.CurrentTable(stmt), clause.Column{Name: field.DBName}, fileType, clause.Column{Name: field.DBName}, fileType).Error; err != nil {
 						return err