Prefer `registered` method over ivar in Repository
Prefer `registered` method over ivar in Repository

This fixes a possible bug which cannot be spec'ed yet:

    require 'rom'

    p ROM::Repository[:foo]

fails with

/rom/lib/rom/repository.rb:75:in `[]': undefined method `detect' for nil:NilClass (NoMethodError)
  from foo.rb:4:in `<main>'

endash/rom,rom-rb/rom,pdswan/rom,rom-rb/rom,pvcarrera/rom,rom-rb/rom,dekz/rom,jeremyf/rom,Snuff/rom,dcarral/rom,denyago/rom,pxlpnk/rom,kwando/rom,cored/rom,vrish88/rom