Fix warnings, improve performance (#1665)

* Fix warnings, improve performance

`QueryResult.Items` is now a `IReadOnlyList` so we don't need to
allocate a new `Array` when we have a `List` (and `Items` shouldn't need to
be mutable anyway)

* Update Providers .csproj to latest C#

* Remove extra newline from DtoService.cs

* Remove extra newline from UserLibraryService.cs

MediaBrowser.Common/Extensions/CollectionExtensions.cs

@@ -10,5 +10,22 @@ namespace MediaBrowser.Common.Extensions
             dictionary.TryGetValue(key, out var ret);
             return ret;
         }
+
+        // REVIEW: Inline?
+        /// <summary>
+        /// Copies all the elements of the current collection to the specified list
+        /// starting at the specified destination array index. The index is specified as a 32-bit integer.
+        /// </summary>
+        /// <param name="source">The current collection that is the source of the elements.</param>
+        /// <param name="destination">The list that is the destination of the elements copied from the current collection.</param>
+        /// <param name="index">A 32-bit integer that represents the index in <c>destination</c> at which copying begins.</param>
+        /// <typeparam name="T"></typeparam>
+        public static void CopyTo<T>(this IReadOnlyCollection<T> source, IList<T> destination, int index = 0)
+        {
+            foreach (T item in source)
+            {
+                destination[index++] = item;
+            }
+        }
     }
 }