Bug: register_qty do not works on the read_holding_registers()

[beyonlo]

This bug was extracted from PR #10 report, in this reply:

The register_qty do not works on the read_holding_registers():

>>> host.read_holding_registers(slave_addr=10, starting_addr=93, register_qty=1, signed=False)
(19,)
>>> host.read_holding_registers(slave_addr=10, starting_addr=93, register_qty=2, signed=False)
(19,) <-- I believe that here should be (19, 786) - register address 93 and 94
>>> host.read_holding_registers(slave_addr=10, starting_addr=94, register_qty=2, signed=False)
(786,)
>>> 

Using this register_definitions:
{'COILS': {'EXAMPLE_COIL': {'register': 123, 'len': 3, 'val': [1, 1, 1]}}, 'HREGS': {'EXAMPLE_HREG': {'register': 93, 'len': 4, 'val': 19}, 'EXAMPLE_HREG_2': {'register': 94, 'len': 2, 'val': 786}}, 'ISTS': {'EXAMPLE_ISTS': {'register': 67, 'len': 1, 'val': [1, 0, 0, 1]}}, 'IREGS': {'EXAMPLE_IREG': {'register': 10, 'len': 2, 'val': 60001}}}

Testing a different register_definitions model:

If I change the register_definitions to this:
{'COILS': {'EXAMPLE_COIL': {'register': 123, 'len': 3, 'val': [1, 1, 1]}}, 'HREGS': {'EXAMPLE_HREG': {'register': 93, 'len': 1, 'val': [19, 23494]}}, 'ISTS': {'EXAMPLE_ISTS': {'register': 67, 'len': 1, 'val': [1, 0, 0, 1]}}, 'IREGS': {'EXAMPLE_IREG': {'register': 10, 'len': 2, 'val': 60001}}}

I have this results:
*1. The register_qty still not works, but when I try to read, address 93, 94 came together
*2. When I try to read/write address 94, I have error
*3. When I try to write address 93, the value on address 94 is lost.

>>> host.read_holding_registers(slave_addr=10, starting_addr=93, register_qty=1, signed=False)
(19, 23494) <-- (1) register_qty is 1, but came 2 address: the 93 and 94
>>> host.read_holding_registers(slave_addr=10, starting_addr=94, register_qty=1, signed=False) <-- (2) 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "umodbus/tcp.py", line 118, in read_holding_registers
  File "umodbus/tcp.py", line 89, in _send_receive
  File "umodbus/tcp.py", line 74, in _validate_resp_hdr
ValueError: slave returned exception code: 131
>>> 
>>> host.read_holding_registers(slave_addr=10, starting_addr=93, register_qty=1, signed=False)
(19, 23494)
>>> 
>>> host.write_single_register(slave_addr=10, register_address=94, register_value=555, signed=False) <-- (2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "umodbus/tcp.py", line 157, in write_single_register
  File "umodbus/tcp.py", line 89, in _send_receive
  File "umodbus/tcp.py", line 74, in _validate_resp_hdr
ValueError: slave returned exception code: 134
>>> host.write_single_register(slave_addr=10, register_address=93, register_value=15, signed=False)
True
>>> host.read_holding_registers(slave_addr=10, starting_addr=93, register_qty=2, signed=False)
(15,) <-- (3) Check que value 23494 was lost
>>> 

Well, in the second test I think that I broken the register_refinitions logic, but I do not know what is the correct rules for it. What is the correct usage on the register definitions?

[brainelectronics]

See Modbus Application Protocol section 6.3

Relates to #18