fix compatability for Xcode 11

[dvshelley]

New Pull Request Checklist

• [ x] I have read and understood the CONTRIBUTING guide

• [x ] I have read the Documentation

• [ x] I have searched for a similar pull request in the project and found none

• [x ] I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• [x ] I have added the required tests to prove the fix/feature I am adding

• [ x] I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

This merge request fixes / refers to the following issues: ...

Pull Request Description

This backwards-compatible change fixes SDWebImage so it will compile under Xcode 11.
...

[dreampiggy]

I'm using Xcode 11-beta. This API is still available under iOS 13 SDK. Does this cause problem ?

[dreampiggy]

This is not because of API changes. This is because the Default Xcode Warning Flags.

In Xcode 11, the default warning flags turn on the -Werror,-Wshorten-64-to-32. Which treat this implict convert as a Error but not a Warning.

However, in Xcode 10, this does not trigger as Error.

Though this fix is OK. I found it's a little strange, because SDWebImage always fix all the warnings from Xcode before releases. This warning should not exist at that time.

@kinarob What about turn SDWebImage warning flags as -Werror, -Weverything ? (Treat all Warnings as Errors)

[dreampiggy]

I found the reason. Seems the clang compiler bug of warning flags before Xcode 11. It will not cause a warning when using the typedef NS_ENUM(NSInteger) enum, but only works for the NSInteger type.

See the test case:

Xcode 10

Xcode 11

[dreampiggy]

I'll merge this now. And also with SDWebImage 4.x branch (have the same issue). And then release two patch version for 3.x version and 4.x version