4.X: fix bitfield off-by-one encoding

[yuzawa-san]

this addresses the feedback done in #83 (comment)

from one of the tcf specs:

PurposeConsent
Bitfield(24)

One bit for each Purpose:
1 Consent
0 No Consent
The user’s consent value for each Purpose established on the legal basis of consent.

The Purposes are numerically identified and published in the Global Vendor List. From left to right, Purpose 1 maps to the 0th bit, purpose 24 maps to the bit at index 23. Special Purposes are a different ID space and not included in this field.