Follow-up from "Repository: Add filter_merge_requests method"

The following discussion from !248 (merged) should be addressed:

• @seblat started a discussion: (+3 comments)

  This will by default only return the MRs with the state 'opened'. The former implementation returned all MRs regardless of the state. You should pass the parameter state = 'all'

  TBH it's the same problem with filter_issues. This might be a bug? @sils ?

  answer by @sils : Returning only the opened MRs might not what you expect, when you call the MR property. Yet it's being used like this atm and we keep it for compatibility reasons.

  I've commented in the inteface, that the docstring should be changed.

  I've openend an issue to change the docstring of 'filter_issues'