Add CaseIterable to OpenGraphMetadata

[robbiet480]

This adds the new in Swift 4.2 CaseIterable to the OpenGraphMetadata enum to allow iterating all possible cases of the enum. For those not on Swift 4.2 yet, there is a compatibility protocol I found on StackOverflow.

This allows me to iterate the entire OpenGraph dictionary by its keys without needing to make the dictionary public.

[satoshi-takano]

Hi @robbiet480 , Thanks for your pull request!
Could you please update osx_image field in .travis.yml to osx_image: xcode10 because the Travis CI build has failed. Other changes are great 👍 Thanks.

[robbiet480]

@satoshi-takano Did that, looks like tests are now failing because "Maximum number of login attempts exceeded. Please try again later." while getting OHHTTPStubs.

[satoshi-takano]

@robbiet480 Thanks for fixing! In fact, the test result also says "Testing failed: Property 'allCases' must be declared public because it matches a requirement in public protocol 'CaseIterable'". So could you please mark allCases as public? Thanks.

[satoshi-takano]

@robbiet480 I've merged your pull request and will resolve the CI error myself. Thanks for your pull request!

[robbiet480]

@satoshi-takano Sorry about that, got suddenly busy last week. Thanks for merging and fixing!