sqlite composite primary key

Calling
```
table.primary(['foo', 'order_id']);
```
will generate
```
create table "users" ("foo" varchar(255), "order_id" varchar(255), primary key ("foo,order_id"))
```
instead of
```
create table "users" ("foo" varchar(255), "order_id" varchar(255), primary key ("foo", "order_id"))
```

This patch fixes this problem.