Removed "has" method usage to be cache-compatible

refs https://github.com/TryGhost/Toolbox/issues/364

- Cache adapter does not have a "has" method, so using "get" instead to achieve the same to have full compatibility. Should allow to just drop in the cache adapter implementation instead of current Map instance.

ghost/core/core/server/lib/image/cached-image-size-from-url.js

@@ -28,7 +28,7 @@ class CachedImageSizeFromUrl {
         }
 
         // image size is not in cache
-        if (!this.cache.has(url)) {
+        if (!this.cache.get(url)) {
             return this.imageSize.getImageSizeFromUrl(url).then((res) => {
                 this.cache.set(url, res);