[Objective-C] No longer ignore `xcworkspace` documents.

Workspaces are real documents that can be opened, just like `xcodeproj`
documents. These documents are also generated/used by CocoaPods to
configure a project’s dependencies. For these reasons, there does not
seem a good reason to ignore these any longer.
Merge pull request #508 from alloy/master

[Objective-C] No longer ignore `xcworkspace` documents.