WIP: Support RubyGems without -doc subpackage

Hi,

I continue on making auto-updates of specs possible. This PR deals with support for specs without -doc subpackage. It also fixes a few things on the way, namely:

* rpmize: don't add %doc if it was added already (this was introduced by one of my commits)
* delete the @update_gem mechanism as otherwise you cannot update to the specified .gem (it would always ask for upstream)
* meta[:pkg_files] has to be set (maybe we can ensure it differently, ideas welcomed)
* include files in -doc subpackage only if it exists
* correctly count indexes if -doc subpackage is missing and/or requires are missing (for fedora 21 and higher we auto-generate the requires and provides)
* add specs for updating spec files

This PR is not completely ready. I want to adjust (format) the test and test against the expected specs which should ensure that indexes are correct as well. Please review.