GHOrganization::listMembersWithFilter does not create correct URL

[Sage-Pierce]

Describe the bug
When trying to list members with a certain filter (or role), a malformed URL is created and GitHub returns Not Found

To Reproduce
Steps to reproduce the behavior:

1. Try to list members of an org with any filter by calling GHOrganization.listMembersWithFilter("all")

Expected behavior
Proper URL should be created and a list of members returned

Desktop (please complete the following information):

• OS: MacOS High Sierra

Additional context
Would also be a good opportunity to allow the role query parameter