Fix adding extra unique constraint to columns that are already unique

[kvch]

Previously, pgroll failed to add unique constraint to columns that were already included in another unique constraint.

Now this problem is fixed.

Related to #227

[andrew-farries]

Is it possible to add a test case for the problem that this fixes?

Based on the PR description I tried (using the migrations in this Gist) to add a UNIQUE constraint involving a column that was already UNIQUE and it worked as expected:

After completing both migrations the schema was:

+----------+------------------------+-----------------------------------------------------+----------+--------------+-------------+
| Column   | Type                   | Modifiers                                           | Storage  | Stats target | Description |
|----------+------------------------+-----------------------------------------------------+----------+--------------+-------------|
| id       | integer                |  not null default nextval('items_id_seq'::regclass) | plain    | <null>       | <null>      |
| name     | character varying(255) |  not null                                           | extended | <null>       | <null>      |
| producer | character varying(255) |  not null                                           | extended | <null>       | <null>      |
+----------+------------------------+-----------------------------------------------------+----------+--------------+-------------+
Indexes:
    "items_pkey" PRIMARY KEY, btree (id)
    "items_name_key" UNIQUE CONSTRAINT, btree (name)
    "unique_name_producer" UNIQUE CONSTRAINT, btree (name, producer)

This was using commit daba767

[kvch]

Haha, yes. I will redo the example. The test case I put together seemed too complex, but apparently I simplified to too much :D