Fix exception updating CreatedDate when file exists

Author: tomfulton

src/UmbracoFileSystemProviders.Azure/AzureFileSystem.cs

@@ -216,7 +216,14 @@ public void AddFile(string path, Stream stream, bool overrideIfExists)
                 }
 
                 // Store the creation date in meta data.
-                blockBlob.Metadata.Add("CreatedDate", created.ToString(CultureInfo.InvariantCulture));
+                if (blockBlob.Metadata.ContainsKey("CreatedDate"))
+                {
+                    blockBlob.Metadata["CreatedDate"] = created.ToString(CultureInfo.InvariantCulture);
+                }
+                else
+                {
+                    blockBlob.Metadata.Add("CreatedDate", created.ToString(CultureInfo.InvariantCulture));
+                }
                 blockBlob.SetMetadata();
             }
             catch (Exception ex)