HierarchicalKey::derivePath() does not accept absolute paths.

[dan-da]

Hi, thanks for taking over maintenance on this library!

It seems ProtonMail/bitcoin-php main branch was forked from bit-wasp/bitcoin-php 1.0 branch rather than master. As such, it is missing this commit that fixes an issue exposed by my tool hd-wallet-derive, and possibly other commits also.

For background, see:

• https://github.com/Bit-Wasp/bitcoin-php/issues/850
• https://github.com/Bit-Wasp/bitcoin-php/pull/872