optional flag to not throw an exception on Block.from_hash(h) in tests

  we can do stuff like this in tests now:

    h['tx'][1]['in'].first['scriptSig'][0] = '1' # break signature
    b = Bitcoin::P::Block.from_hash(h, false)
    b.recalc_mrkl_root and b.recalc_block_hash
    Bitcoin::P::Block.from_hash(b.to_hash)