ModbusRequest::parse_slice() applied offset is wrong for write functions

[Achtuur]

Problem

When calling client::MobusRequest::parse_slice() using a buffer of data from a write request (ie modbus functions 0x6 or 0x10), the returned slice skips the high byte of the read register. In parse_slice(), an offset of 3 is applied to the start of the frame, which assumes there is always 3 bytes before the actual data bytes. This is true for reads, as there is a byte that indicates the number of data bytes to follow, but write functions are missing this byte. While this is not per se a huge issue, since you usually don't care about the response to a write function, it's still incorrectly parsed.

Example

use rmodbus::client::ModbusRequest;
let mut request = ModbusRequest::new(11, ModbusProto::Rtu);
let mut buf = Vec::new();

// buf now contains [0xB, 0x6, 0x0, 0x1, 0x0, 0x3, 0x98, 0xA1]
request.generate_set_holding(1, 2, &mut buf)?;

let expected_response = buf; // write functions echo the request
// parsed should have the written to registers and written value
// which is [0x0, 0x1, 0x0, 03] in this case
let parsed = request.parse_slice(&expected_response)

// this fails, as parsed is equal to [0x1, 0x0, 0x3] (skips the first byte)
assert_eq!(parsed, [0x0, 0x1, 0x0, 0x3]);

Proposed solution

Instead of always adding 3 to the start of the frame, a check should be done on the function code. If the function code corresponds to a write function, the offset should be 2 instead.

[Achtuur]

created pr at #27