Use dispatch_barrier_sync (AFNetworking#4474)

This should hopefully avoid any problems where we attempt to write an
HTTP header to a deallocated header dictionary.

See AFNetworking#3891
See https://github.com/AFNetworking/AFNetworking/issues/3636
Modified from tmm1@32760e7

Fixes AFNetworking#1391

AFNetworking/AFURLRequestSerialization.m

@@ -313,7 +313,7 @@ - (NSDictionary *)HTTPRequestHeaders {
 - (void)setValue:(NSString *)value
 forHTTPHeaderField:(NSString *)field
 {
-    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
+    dispatch_barrier_sync(self.requestHeaderModificationQueue, ^{
         [self.mutableHTTPRequestHeaders setValue:value forKey:field];
     });
 }
@@ -335,7 +335,7 @@ - (void)setAuthorizationHeaderFieldWithUsername:(NSString *)username
 }
 
 - (void)clearAuthorizationHeader {
-    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
+    dispatch_barrier_sync(self.requestHeaderModificationQueue, ^{
         [self.mutableHTTPRequestHeaders removeObjectForKey:@"Authorization"];
     });
 }