Update the coders manager initializer, without the useless mutable copy

Update the comments

SDWebImage/SDImageCachesManager.m

@@ -36,8 +36,8 @@
         self.containsOperationPolicy = SDImageCachesManagerOperationPolicySerial;
         self.clearOperationPolicy = SDImageCachesManagerOperationPolicyConcurrent;
         // initialize with default image caches
-        self.caches = @[[SDImageCache sharedImageCache]];
-        self.cachesLock = dispatch_semaphore_create(1);
+        _caches = @[[SDImageCache sharedImageCache]];
+        _cachesLock = dispatch_semaphore_create(1);
     }
     return self;
 }

SDWebImage/SDImageCodersManager.h

@@ -17,10 +17,10 @@
  
  Note: the `coders` getter will return the coders in their reversed order
  Example:
- - by default we internally set coders = `IOCoder`, `GIFCoder`
- - calling `coders` will return `@[IOCoder, GIFCoder]`
+ - by default we internally set coders = `IOCoder`, `GIFCoder`, `APNGCoder`
+ - calling `coders` will return `@[IOCoder, GIFCoder, APNGCoder]`
  - call `[addCoder:[MyCrazyCoder new]]`
- - calling `coders` now returns `@[IOCoder, GIFCoder, MyCrazyCoder]`
+ - calling `coders` now returns `@[IOCoder, GIFCoder, APNGCoder, MyCrazyCoder]`
  
  Coders
  ------

SDWebImage/SDImageCodersManager.m

@@ -31,8 +31,7 @@
 - (instancetype)init {
     if (self = [super init]) {
         // initialize with default coders
-        NSMutableArray<id<SDImageCoder>> *mutableCoders = [@[[SDImageIOCoder sharedCoder], [SDImageGIFCoder sharedCoder], [SDImageAPNGCoder sharedCoder]] mutableCopy];
-        _coders = [mutableCoders copy];
+        _coders = @[[SDImageIOCoder sharedCoder], [SDImageGIFCoder sharedCoder], [SDImageAPNGCoder sharedCoder]];
         _codersLock = dispatch_semaphore_create(1);
     }
     return self;

SDWebImage/SDImageLoadersManager.m

@@ -30,8 +30,8 @@
     self = [super init];
     if (self) {
         // initialize with default image loaders
-        self.loaders = @[[SDWebImageDownloader sharedDownloader]];
-        self.loadersLock = dispatch_semaphore_create(1);
+        _loaders = @[[SDWebImageDownloader sharedDownloader]];
+        _loadersLock = dispatch_semaphore_create(1);
     }
     return self;
 }