[#289] Add coverage for eliminating default value tag (#290)

* [#289] Add coverage for eliminating default value tag

* [#289] wip

migrator.go

@@ -393,10 +393,16 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 								return err
 							}
 						} else {
-							if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? DROP DEFAULT", m.CurrentTable(stmt), clause.Column{Name: field.DBName}, clause.Expr{SQL: field.DefaultValue}).Error; err != nil {
+							if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? DROP DEFAULT", m.CurrentTable(stmt), clause.Column{Name: field.DBName}).Error; err != nil {
 								return err
 							}
 						}
+					} else if !field.HasDefaultValue {
+						// case - as-is column has default value and to-be column has no default value
+						// need to drop default
+						if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? DROP DEFAULT", m.CurrentTable(stmt), clause.Column{Name: field.DBName}).Error; err != nil {
+							return err
+						}
 					}
 				}
 				return nil