removes undue reported error when forwarding (#1134)

* removes undue reported error when forwarding

cf #1110 (comment)

this removes the error, when running a TCP server forwarding a serial server :
```
17:10:21 ERROR mixin:113 Please do not use unit=, convert to slave=.
````

* homogeneisation

ReadRegistersRequestBase cf ResponseBase

Author: alexandrecuer

pymodbus/datastore/remote.py

@@ -73,7 +73,7 @@ def __build_mapping(self):
         """Build the function code mapper."""
         kwargs = {}
         if self.unit:
-            kwargs["unit"] = self.unit
+            kwargs["slave"] = self.unit
         self.__get_callbacks = {
             "d": lambda a, c: self._client.read_discrete_inputs(  # pylint: disable=unnecessary-lambda
                 a, c, **kwargs

pymodbus/register_read_message.py

@@ -49,7 +49,7 @@ def __str__(self):
 
         :returns: A string representation of the instance
         """
-        return f"ReadRegisterRequest ({self.address},{self.count})"
+        return f"{self.__class__.__name__} ({self.address},{self.count})"
 
 
 class ReadRegistersResponseBase(ModbusResponse):