* Add StringEx.ToByteArray(...)
* Add Serialization.SerializeDataItem(DataItem)
Supports StringEx VarType or offloads to SerializeValue method.
* Use SerializeDataItem in S7WriteMultiple
* Assume string length without header in StringEx.ToByteArray
VarTypeToByteLength already assumed that StringEx declared count for
the number of characters without the header, this now matches that
behavior.
* Add unit tests for StringEx conversions
* Fix incorrect value passed to Encoding.GetBytes
The length must actually be within string limits.
