Remove redundant allocation in Rlp.Encode<T> when input is already Rlp

src/Nethermind/Nethermind.Core.Test/RlpTests.cs

@@ -246,6 +246,16 @@ public void Long_and_big_integer_encoded_the_same(long value)
             Assert.That(rlpBigInt.Bytes, Is.EqualTo(rlpLong.Bytes));
         }
 
+        [Test]
+        public void Encode_generic_with_Rlp_input_preserves_original_bytes()
+        {
+            Rlp original = Rlp.Encode(255L);
+            Rlp reEncoded = Rlp.Encode<Rlp>(original);
+
+            Assert.That(reEncoded.Bytes, Is.EqualTo(original.Bytes));
+            Assert.That(reEncoded, Is.SameAs(original));
+        }
+
         [TestCase(true)]
         [TestCase(false)]
         public void RlpContextWithSliceMemory_shouldNotCopyUnderlyingData(bool sliceValue)

src/Nethermind/Nethermind.Serialization.Rlp/Rlp.cs

@@ -328,8 +328,7 @@ public static Rlp Encode<T>(T item, RlpBehaviors behaviors = RlpBehaviors.None)
         {
             if (item is Rlp rlp)
             {
-                RlpStream stream = new(LengthOfSequence(rlp.Length));
-                return new(stream.Data.ToArray());
+                return rlp;
             }
 
             IRlpStreamDecoder<T>? rlpStreamDecoder = GetStreamDecoder<T>();