Don't try to drop a default twice when changing a column's type on Mi…

…crosoft SQL Server

This broke when the code to drop a default before changing a default
was added a couple days ago.

lib/sequel/adapters/shared/mssql.rb

@@ -279,7 +279,7 @@ def alter_table_sql(table, op)
  end
  end
  sqls << "ALTER TABLE #{quote_schema_table(table)} ALTER COLUMN #{column_definition_sql(op)}"
- sqls << alter_table_sql(table, op.merge(:op=>:set_column_default, :default=>default)) if default
+ sqls << alter_table_sql(table, op.merge(:op=>:set_column_default, :default=>default, :skip_drop_default=>true)) if default
  sqls
  when :set_column_null
  sch = schema(table).find{|k,v| k.to_s == op[:name].to_s}.last
@@ -291,7 +291,7 @@ def alter_table_sql(table, op)
  "ALTER TABLE #{quote_schema_table(table)} ALTER COLUMN #{quote_identifier(op[:name])} #{type_literal(:type=>type)} #{'NOT ' unless op[:null]}NULL"
  when :set_column_default
  sqls = []
- add_drop_default_constraint_sql(sqls, table, op[:name])
+ add_drop_default_constraint_sql(sqls, table, op[:name]) unless op[:skip_drop_default]
  sqls << "ALTER TABLE #{quote_schema_table(table)} ADD CONSTRAINT #{quote_identifier("sequel_#{table}_#{op[:name]}_def")} DEFAULT #{literal(op[:default])} FOR #{quote_identifier(op[:name])}"
  else
  super(table, op)