Swift 3.0: Fix Xcode 8 GM compatibility

[ikesyo]

No description provided.

[mxcl]

We want to keep the single #if block, this was a design decision in previous PRs, could you amend the patch conforming to the previous style? Thanks.

[ikesyo]

Rebased

[mxcl]

Many thanks.

[AliSoftware]

👍 thanks for the taking care of this fix 👌

[AliSoftware]

5.2.1 is out, with this fix included 🎉

• I cherry-picked this fix from swift3 in master, so that people using CocoaPods can still point to pod 'OHHTTPStubs, '~> 5.2' and get that change too if they're using Xcode 8 GM
• I rebased the swift3 branch on top of master so that the only commit making swift3 differ from master is still SWIFT_VERSION=3.0 only, as this branch's only purpose is for Carthage users only

For Swift 2.2 / 2.3 users, this 5.2.1 doesn't have much change compared to 5.2.0 (except README & docs typos) but I prefer keeping all my versions in sync to avoid confusions.