rdoc --coverage-report bombs on class/module alias

[alindeman]

Given a file class_alias.rb:

module Test
  class Foo
  end
end

Foo = Test::Foo

Running rdoc -C class_alias.rb will bomb with this stack trace:

undefined method `full_name' for nil:NilClass
/home/andy/workspace/rdoc/lib/rdoc/stats.rb:219:in `report'
    /home/andy/workspace/rdoc/lib/rdoc/context.rb:651:in `each_constant'
    /home/andy/workspace/rdoc/lib/rdoc/context.rb:651:in `each'
    /home/andy/workspace/rdoc/lib/rdoc/context.rb:651:in `each_constant'
    /home/andy/workspace/rdoc/lib/rdoc/stats.rb:217:in `report'
    /home/andy/workspace/rdoc/lib/rdoc/stats.rb:182:in `each'
    /home/andy/workspace/rdoc/lib/rdoc/stats.rb:182:in `report'
    /home/andy/workspace/rdoc/lib/rdoc/rdoc.rb:421:in `document'
    bin/rdoc:15

My commit will ignore undocumented class/modules aliases in the report printing routine. The original class is the only one that needs to appear in the report.

[drbrain]

Applied.

I also wrote a test and added some TODOs for the appearance of constant aliases in the summary and report. Since they're usually a tool of programmer convenience I don't think they should be counted against the documentation summary.