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

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

* Fix

* Regenerated test code

src/idl_gen_csharp.cpp

@@ -845,6 +845,8 @@ class CSharpGenerator : public BaseGenerator {
         code += "Bytes() { return ";
         code += "__p.__vector_as_arraysegment(";
         code += NumToString(field.value.offset);
+        code +=
+            ", " + NumToString(SizeOf(field.value.type.VectorType().base_type));
         code += "); }\n";
         code += "#endif\n";