Update for Xcode 11#1

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p0dee wants to merge 5 commits intomasterfrom

update-for-swift5

Commits on Apr 16, 2020

Make code compatible with Swift 4 to enable to build using Xcode 11.x
Takeshi Tanaka
committed
Remove unnecessary `public` access modifiers
Takeshi Tanaka
committed
Change SWIFT_VERSION to 4.0
Takeshi Tanaka
committed
Fix tests to pass them
Takeshi Tanaka
committed

Commits on Apr 17, 2020

Fix to handle NSNull as nil
Takeshi Tanaka
committed