Handle prefork hash correctly

[h4x3rotab]

block100.getHash().reverse().toString('hex');

It produces the wrong hash for block 100: 7644500d651d8db203d8dfdb7053da2cffe8dffcb9a1bc115da63e047dfb7c17

The reason is that it tries to serialize a prefork Bitcoin block in the BTG block header manner. It will produce wrong hash and thus can't pass the target check as well.

Note that explorer.bitcoingold.org also has the same problem (so don't see it as the ground truth).

Solution: check the block height before doing serialization just like how BTG Core handles it.

Test vector: block 100
Hash: 000000007bc154e0fa7ea32218a72fe2c1bb9f86cf8c9ebf9a715ed27fdb229a
Height: 100
Hex:

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

[Vutov]

#5