add hashCode method

src/ImageSharp/Metadata/Profiles/Exif/Values/EncodedString.cs

@@ -46,6 +46,10 @@ namespace SixLabors.ImageSharp.Metadata.Profiles.Exif
             return this.Text == other.Text && this.Code == other.Code;
         }
 
+        /// <inheritdoc/>
+        public override int GetHashCode() =>
+            HashCode.Combine(this.Text, this.Code);
+
         /// <inheritdoc/>
         public override string ToString() => this.Text;
     }