The SchemaLoader raises an error on GitLab's schema.rb

[pocke]

I've added the schema loader in #197, but I found an error with GitLab's db/schema.rb.
I'll send a pull request as soon as possible, probably tonight.

---

Expected behavior

No errors during parsing db/schema.rb.

Actual behavior

An error occurred (at least).

Steps to reproduce the problem

$ git clone https://github.com/gitlabhq/gitlabhq
$ cd gitlabhq/
$ ruby -rparser/ruby26 -rrubocop-rails -e 'RuboCop::Rails::SchemaLoader.load 2.6'
/home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/parser-2.7.0.2/lib/parser/lexer.rb:10972: warning: assigned but unused variable - testEof
/home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:134:in `build_columns': undefined method `values' for s(:str, "lower((extern_uid)::text), provider"):RuboCop::AST::StrNode (NoMethodError)
Did you mean?  value
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:125:in `initialize'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:75:in `new'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:75:in `block in build_indices'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:85:in `block in each_content'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:84:in `each'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:84:in `each_content'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:71:in `each'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:71:in `map'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:71:in `build_indices'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:52:in `initialize'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:27:in `new'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:27:in `block in build!'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:37:in `block in each_table'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:34:in `each'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:34:in `each_table'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:26:in `build!'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader/schema.rb:12:in `initialize'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader.rb:34:in `new'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader.rb:34:in `load!'
	from /home/pocke/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rubocop-rails-2.4.2/lib/rubocop/rails/schema_loader.rb:18:in `load'
	from -e:1:in `<main>'

The cause is the following line:

    t.index "lower((extern_uid)::text), provider", name: "index_on_identities_lower_extern_uid_and_provider"

https://github.com/gitlabhq/gitlabhq/blob/dc560bd238764df34382017e23f2667ade0c7a2d/db/schema.rb#L2017

RuboCop version

master branch.