Bug when writing more than 12 Boolean variables

[FiloCara]

Hello

There is a bug which prevent to write more than 12 Boolean variables at the same time.
This issue is strictly related to the one I opened a while ago for the node-red-contrib-s7 package (st-one-io/node-red-contrib-s7#90)

Steps to reproduce:
Write more than 12 Boolean variables at the same time using the writeItems method of the S7ItemGroup class. This raises a request timeout.

Problem:
After investigating, the problem seems related to the way the reqItemLength variable is computed.

A writing_var (s7) request for multiple Boolean variables requires the following bytes:

• HEADER: 10 bytes
• PARAMETERS: 2 bytes + 12 bytes per variable
• DATA: 4 bytes per variable + 1 byte per variable + 1 extra fill byte per variable (something like a separator character)

The reqItemLength does not take into account the fill byte.
I'm going to open a pull request to fix it.

Filippo