Fix HTTP Header key field nullable

SDWebImage/SDAnimatedImageView.h

@@ -61,7 +61,7 @@
  You can specify a runloop mode to let it rendering.
  Default is NSRunLoopCommonModes on multi-core iOS device, NSDefaultRunLoopMode on single-core iOS device
  */
-@property (nonatomic, copy, nonnull) NSRunLoopMode runLoopMode NS_AVAILABLE_IOS(3_1);
+@property (nonatomic, copy, nonnull) NSRunLoopMode runLoopMode;
 
 @end
 

SDWebImage/SDWebImageDownloader.m

@@ -133,6 +133,9 @@ static void * SDWebImageDownloaderContext = &SDWebImageDownloaderContext;
 }
 
 - (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field {
+    if (!field) {
+        return;
+    }
     NSMutableDictionary *mutableHTTPHeaders = [self.HTTPHeaders mutableCopy];
     if (value) {
         [mutableHTTPHeaders setObject:value forKey:field];