Move assertions so they are valid. (#1261)

If PIN_ANIMATED is enabled, the creation of a PINRemoteImageManager
will create a sharedDownloader also. Thus we cannot assert that it
doesn't exist. We will move this assertion up to the methods that
create the preconfiguredSharedManager and before said manager is
allocated.

Source/Details/ASPINRemoteImageDownloader.mm

@@ -115,13 +115,15 @@ + (ASPINRemoteImageDownloader *)sharedDownloader NS_RETURNS_RETAINED
 
 + (void)setSharedImageManagerWithConfiguration:(nullable NSURLSessionConfiguration *)configuration
 {
+  NSAssert(sharedDownloader == nil, @"Singleton has been created and session can no longer be configured.");
   PINRemoteImageManager *sharedManager = [self PINRemoteImageManagerWithConfiguration:configuration imageCache:nil];
   [self setSharedPreconfiguredRemoteImageManager:sharedManager];
 }
 
 + (void)setSharedImageManagerWithConfiguration:(nullable NSURLSessionConfiguration *)configuration
                                     imageCache:(nullable id<PINRemoteImageCaching>)imageCache
 {
+  NSAssert(sharedDownloader == nil, @"Singleton has been created and session can no longer be configured.");
   PINRemoteImageManager *sharedManager = [self PINRemoteImageManagerWithConfiguration:configuration imageCache:imageCache];
   [self setSharedPreconfiguredRemoteImageManager:sharedManager];
 }
@@ -131,7 +133,6 @@ + (void)setSharedImageManagerWithConfiguration:(nullable NSURLSessionConfigurati
 + (void)setSharedPreconfiguredRemoteImageManager:(PINRemoteImageManager *)preconfiguredPINRemoteImageManager
 {
   NSAssert(preconfiguredPINRemoteImageManager != nil, @"setSharedPreconfiguredRemoteImageManager requires a non-nil parameter");
-  NSAssert(sharedDownloader == nil, @"Singleton has been created and session can no longer be configured.");
   NSAssert1(sharedPINRemoteImageManager == nil, @"An instance of %@ has been set. Either configuration or preconfigured image manager can be set at a time and only once.", [[sharedPINRemoteImageManager class] description]);
 
   dispatch_once(&shared_init_predicate, ^{