Keeping both owned and unowned sessions around as part of the operation.

We need different ownership for both because we need to invalidate the owned session upon completion.
The unowned session is wet to avoid cyclical references.

@@ -29,9 +29,9 @@ NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinis
 
 // This is weak because it is injected by whoever manages this session. If this gets nil-ed out, we won't be able to run
 // the task associated with this operation
-@property (weak, nonatomic) NSURLSession *session;
-// This is set to NO if we're using an injected NSURLSession, and to YES otherwise
-@property (assign, nonatomic) BOOL ownSession;
+@property (weak, nonatomic) NSURLSession *unownedSession;
+// This is set if we're using not using an injected NSURLSession. We're responsible of invalidating this one
+@property (strong, nonatomic) NSURLSession *ownedSession;
 
 @property (strong, nonatomic, readwrite) NSURLSessionTask *dataTask;
 
@@ -82,7 +82,7 @@ NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinis
         _executing = NO;
         _finished = NO;
         _expectedSize = 0;
-        _session = session;
+        _unownedSession = session;
         responseFromCached = YES; // Initially wrong until `- URLSession:dataTask:willCacheResponse:completionHandler: is called or not called
     }
     return self;
@@ -114,24 +114,24 @@ NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinis
             }];
         }
 #endif
-        if (!self.session) {
-            self.ownSession = YES;
-
+        NSURLSession *session = self.unownedSession;
+        if (!self.unownedSession) {
             NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
             sessionConfig.timeoutIntervalForRequest = 15;
-
+            
             /**
              *  Create the session for this task
              *  We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate
              *  method calls and completion handler calls.
              */
-            self.session = [NSURLSession sessionWithConfiguration:sessionConfig
-                                                         delegate:self
-                                                    delegateQueue:nil];
+            self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
+                                                              delegate:self
+                                                         delegateQueue:nil];
+            session = self.ownedSession;
         }
         
+        self.dataTask = [session dataTaskWithRequest:self.request];
         self.executing = YES;
-        self.dataTask = [self.session dataTaskWithRequest:self.request];
         self.thread = [NSThread currentThread];
     }
     
@@ -213,9 +213,9 @@ NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinis
     self.dataTask = nil;
     self.imageData = nil;
     self.thread = nil;
-    if (self.ownSession) {
-        [self.session invalidateAndCancel];
-        self.session = nil;
+    if (self.ownedSession) {
+        [self.ownedSession invalidateAndCancel];
+        self.ownedSession = nil;
     }
 }