Merge pull request SDWebImage#942 from Krivoblotsky/master

'304 Not Modified' HTTP status code handling.

Author: bpoplauschi

SDWebImage/SDWebImageDownloaderOperation.m

@@ -195,7 +195,9 @@ - (BOOL)isConcurrent {
 #pragma mark NSURLConnection (delegate)
 
 - (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
-    if (![response respondsToSelector:@selector(statusCode)] || [((NSHTTPURLResponse *)response) statusCode] < 400) {
+    
+    //'304 Not Modified' is an exceptional one
+    if ((![response respondsToSelector:@selector(statusCode)] || [((NSHTTPURLResponse *)response) statusCode] < 400) && [((NSHTTPURLResponse *)response) statusCode] != 304) {
         NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
         self.expectedSize = expected;
         if (self.progressBlock) {
@@ -205,8 +207,16 @@ - (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLRespon
         self.imageData = [[NSMutableData alloc] initWithCapacity:expected];
     }
     else {
-        [self.connection cancel];
-
+        NSUInteger code = [((NSHTTPURLResponse *)response) statusCode];
+        
+        //This is the case when server returns '304 Not Modified'. It means that remote image is not changed.
+        //In case of 304 we need just cancel the operation and return cached image from the cache.
+        if (code == 304) {
+            [self cancelInternal];
+        } else {
+            [self.connection cancel];
+        }
+        
         [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:nil];
 
         if (self.completedBlock) {