fix: alias to a method with call-seq should render properly if the call-seq does not specify the alias

[flavorjones]

Description of the problem

Fixes #792.

If I have this ruby code:

class Foo
  # :call-seq:
  #   foo(a)
  #   foo(a, b)
  #   foo(a, b, c)
  def foo(*args)
  end

  alias :bar :foo
end

The alias bar is rendered poorly:

After this change, it gracefully falls back to using the underlying method's param-seq:

Explanation

The call-seq deduplication introduced in b792230 and refined in 0ead786 returns a blank string if the alias's underlying method does not name the alias in its call-seq.

This change fixes alias call-seq to return nil in this case, which in turn means that the darkfish template will use #param_seq instead of rendering with a missing method-heading div.

This change also backfills test coverage for b792230 and 0ead786 to ensure deduplication still works as expected.