Patch to allow Mysql adapter to properly handle non-standard key on schema dump

http://dev.rubyonrails.org/ticket/9971

This one cost me a few hours trying to debug my test cases when I
really needed to be debugging ActiveRecord.

If you define a table with a key other than id, the db/schema.db will
NOT define any primary key for the table. ActiveRecord::SchemaDumper
has a mechanism to handle this but the mysql adapter wasn't playing
the game.

Please have a look at this patch and verify it.

It passes both the existing mysql regression tests (i.e rake
test_mysql), and the test I added to test this specific use case.

Oops,

It looks like the edge testing setup changed out from under me so that
the patch won't apply to edge.

I'll update the patch once I've figured it out.

I attached a new patch which fixes the test case which was broken
since a switch was made to use ruby schema instead of sql to set up
the test databases for ActiveRecord testing.

The actual code was unaffected.