Block option encoding for 16-bytes blocks

[chrysn]

There's been a bug report in aiocoap about the erroneous encoding of small (16 byte) blockwise options at https://github.com/chrysn/aiocoap/issues/130, and looking at the current txThings code (from whose earlier versions aiocoap originated), I think that https://github.com/mwasilak/txThings/blob/master/txthings/coap.py#L735 is still susceptible to the same problem.

My immediate fix is in https://github.com/chrysn/aiocoap/commit/9bd612dec2aeb7e10837e6190ec6987a28d4f244, and should be applicable to txThings in a similar fashion.