initialise kPNGSignatureData data

in SDImageCache initWithNamespace method.
This ensures the kPNGSignatureData is always initialised even if the we
alloc SDImageCache without using the sharedImageCache singleton

@@ -48,7 +48,6 @@ BOOL ImageDataHasPNGPreffix(NSData *data) {
     static id instance;
     dispatch_once(&once, ^{
         instance = [self new];
-        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];
     });
     return instance;
 }
@@ -61,6 +60,9 @@ BOOL ImageDataHasPNGPreffix(NSData *data) {
     if ((self = [super init])) {
         NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];
 
+        // initialise PNG signature data
+        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];
+
         // Create IO serial queue
         _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);