fix postgresql_adapter index dumping with multiple schemas

Hi,

I've recently updated an old patch to PostgreSQLAdapter#indexes, which
fixes a bug with schema dumping with multiple database schemas
present. The new patch corrects an edge case bug in the original
patch, includes test cases, and has been tested against PostgreSQL
7.4, 8.1 and 8.3.

The ticket is at http://dev.rubyonrails.org/ticket/8659 ; I'd
appreciate any code review, comments, etcetera -- this is my first
attempt at submitting a patch to Rails.

Regards,

Stephen Veiss