Add GetString(ReadOnlySpan<byte> polyfill to Encoding

src/ImageSharp/Common/Extensions/EncoderExtensions.cs

@@ -0,0 +1,34 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+#if !NETCOREAPP2_1
+using System;
+using System.Text;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// Extension methods for the <see cref="Encoder"/> type.
+    /// </summary>
+    internal static unsafe class EncoderExtensions
+    {
+        /// <summary>
+        /// Gets a string from the provided buffer data.
+        /// </summary>
+        /// <param name="encoding">The encoding.</param>
+        /// <param name="buffer">The buffer.</param>
+        /// <returns>The string.</returns>
+        public static string GetString(this Encoding encoding, ReadOnlySpan<byte> buffer)
+        {
+#if NETSTANDARD1_1
+            return encoding.GetString(buffer.ToArray());
+#else
+            fixed (byte* bytes = buffer)
+            {
+                return encoding.GetString(bytes, buffer.Length);
+            }
+#endif
+        }
+    }
+}
+#endif