Added validation back to the copyright frame.

Added some important comments.

src/Id3.Net/Frames/String/CopyrightFrame.cs

@@ -41,9 +41,14 @@ public override string ToString()
 
         protected override void ValidateValue(string value)
         {
-            //TODO: Validate
-            //if (!CopyrightPrefixPattern.IsMatch(value))
-            //    throw new ArgumentException(FrameMessages.Copyright_InvalidFormat, nameof(value));
+            if (!string.IsNullOrEmpty(value) && !CopyrightPrefixPattern.IsMatch(value))
+                throw new ArgumentException(FrameMessages.Copyright_InvalidFormat, nameof(value));
+        }
+
+        internal override string TextValue
+        {
+            get => base.TextValue;
+            set => base.TextValue = !string.IsNullOrEmpty(value) && !CopyrightPrefixPattern.IsMatch(value) ? null : value;
         }
 
         private static readonly Regex CopyrightPrefixPattern = new Regex(@"^\d{4} ");

src/Id3.Net/Frames/TextFrameBase.cs

@@ -77,6 +77,17 @@ public TValue Value
             }
         }
 
+        /// <summary>
+        ///     Deriving classes can override this method to validate the native value being set.
+        ///     <para />
+        ///     If the value is invalid, the method should throw an exception.
+        /// </summary>
+        /// <param name="value">The native value being set.</param>
+        /// <exception cref="Id3Exception">Thrown if the specified native value is invalid.</exception>
+        /// <remarks>
+        ///     Note that in a lot of cases, a native value of null or something that translates to an empty string is considered
+        ///     valid. In such cases, the frame may be unassigned, but the value should still be allowed.
+        /// </remarks>
         protected virtual void ValidateValue(TValue value)
         {
         }

src/Id3.Net/Mp3/Mp3.cs

@@ -120,7 +120,7 @@ public Mp3(byte[] byteStream, Mp3Permissions permissions = Mp3Permissions.Read)
             SetupStream(stream, permissions);
         }
 
-        protected void SetupStream(Stream stream, Mp3Permissions permissions)
+        private void SetupStream(Stream stream, Mp3Permissions permissions)
         {
             if (stream == null)
                 throw new ArgumentNullException(nameof(stream));