Fix internal Linter warnings #trivial

[maicki]

Our internal Linter caught a couple of warnings that this PR will fix. Thanks for @jerrymarino for discovering it:

...
In file included from /Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASDisplayNode.h:27:
/Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASLayoutElement.h:75:1: warning: Using assign for an ObjC pointer type is not valid. Either use strong or weak for a zeroing reference in this case.
@property (nonatomic, assign, readonly) ASLayoutElementStyle *style;
^
...
In file included from /Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/AsyncDisplayKit.h:19:
/Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASDisplayNode.h:649:1: warning: Using a pointer for a scalar type is invalid.
@property (nonatomic, assign, nullable) CGColorRef shadowColor;                // default=opaque rgb black
^
/Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASDisplayNode.h:654:1: warning: Using a pointer for a scalar type is invalid.
@property (nonatomic, assign, nullable) CGColorRef borderColor;                // default=opaque rgb black
^
...
/Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASDisplayNode+Subclasses.h:125:1: warning: Using assign for an ObjC pointer type is not valid. Either use strong or weak for a zeroing reference in this case.
@property (nullable, nonatomic, readonly, assign) ASLayout *calculatedLayout;
^
...
/Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASImageProtocols.h:166:1: warning: block property does not copy the block - use copy attribute instead
@property (nonatomic, strong, readwrite) void (^coverImageReadyCallback)(UIImage *coverImage);
^
...
/Users/jerry/Projects/ios/Platform/include/AsyncDisplayKit/ASImageProtocols.h:212:1: warning: block property does not copy the block - use copy attribute instead
@property (nonatomic, strong, readwrite) dispatch_block_t playbackReadyCallback;
^
6 warnings generated.

[garrettmoon]

Thank you @maicki !

[appleguy]

@maicki @garrettmoon Great find! Any thoughts on why not put copy here explicitly? That would seem to match the style of most of the other changes; if anything, perhaps removing the readwrite specifier is an option too.

The copy semantics as a default are a bit less clear (since it's a special case) than the readwrite semantics (since it's always the case)