Don't reduce lookup context formats

In Rails 3 or 4.0 the you could render an attachment inside a mailer:

```ruby
class Notifier < ActionMailer::Base
  def instructions(user)
    xlsx = render handlers: [:axlsx], template: 'users/mailers/instructions', layout: false
    attachments["user.xlsx"] = {mime_type: Mime::XLSX, content: xlsx}

    mail :to => user.email, :subject => 'Instructions'
  end
end
```

With 4.1 the lookup context reduces the formats list to `:xlsx` and the following template lookup tries to find `instructions.xlsx.erb` instead of `instructions.html.erb`.

This fixes the problem. I assume `lookup_context.formats = [format.to_sym]` was to increase lookup efficiency thereafter. There may be a better way.

All actionview, actionpack, actionmailer, actionsupport tests pass. I'm still getting familiar with the Rails test suite. If someone can suggest where to place templates / test code I'll write one up.