Added will message length to connect method.

[iojan]

First, thanks for this great library!
Current connect method only allows null terminated strings as willMessage payload.
I needed to send binary data, so I added a willMessageLenth argument.
If willMessageLenth is zero, willMessage will be copied as a null terminated string. If bigger than zero, willMessageLenth bytes will be copied form willMessage.
To keep it simple, buffer length check and copy were implemented inside the connect method. I can create a new check macro and copy function if you prefer.

[Illirgway]

Wouldn't it be better to use memcopy / memmove instead of direct byte-to-byte copy loop?

                // const uint8_t* willMsg, uint16_t willMsgSz
                // check willMsg len
                // TODO check overflow
                if (length + 2 + willMsgSz > this->bufferSize) {
                  _client->stop();
                  return false;
                }

                // store msg size prefix
                this->buffer[length+0] = (willMsgSz >> 8);
                this->buffer[length+1] = (willMsgSz & 0xFF);
                length += 2; // count size prefix

                memcpy(&this->buffer[length], willMsg, willMsgSz); // or memmove

                length += willMsgSz;