[C#] Fix truncated ArraySegment<byte> if elementSize != 1 (#6462)

* WIP: Fix returned truncated ArraySegment<byte> if elementSize is not byte

* Fix

* Regenerated test code

tests/FlatBuffers.Test/FlatBuffersExampleTests.cs

@@ -277,6 +277,9 @@ namespace FlatBuffers.Test
             {
                 Assert.IsTrue(monster.GetTestarrayofboolsBytes().HasValue);
             }
+
+            Assert.AreEqual(monster.VectorOfDoublesLength * 8, monster.GetVectorOfDoublesBytes().Value.Count);
+
             #endif
         }