Assert only once we know URL has changed (#247)

We need this in case someone is setting the URL to nil before setting
the image directly. I.e. they have a node which has been used as a network
node or an image node but now they want it to be an image node.

They should then set the URL to nil and then set the image.
*if* it was used as a network node previously, this would work.
If it was previously used as an image node it would historically assert.
This addresses that.

Source/ASNetworkImageNode.mm

@@ -165,14 +165,14 @@ - (void)setURL:(NSURL *)URL resetToDefault:(BOOL)reset
   {
     ASDN::MutexLocker l(__instanceLock__);
 
-    ASDisplayNodeAssert(_imageWasSetExternally == NO, @"Setting a URL to an ASNetworkImageNode after setting an image changes its behavior from an ASImageNode to an ASNetworkImageNode. If this is what you want, set the image to nil first.");
-
-    _imageWasSetExternally = NO;
-
     if (ASObjectIsEqual(URL, _URL)) {
       return;
     }
 
+    ASDisplayNodeAssert(_imageWasSetExternally == NO, @"Setting a URL to an ASNetworkImageNode after setting an image changes its behavior from an ASImageNode to an ASNetworkImageNode. If this is what you want, set the image to nil first.");
+
+    _imageWasSetExternally = NO;
+
     [self _locked_cancelImageDownloadWithResumePossibility:NO];
 
     _imageLoaded = NO;