support zeroed hash root in trie.py about pyethereum HOT 4 CLOSED

This test should pass:
https://github.com/ethereum/pyethereum/blob/master/tests/test_chain.py#L90

Fort this test to pass

• trie must support zero hash roots.
• tries must be initialized with a zero hash root
• i.e. blocks.GENESIS_TX_LIST_ROOT must be "\x00" * 32
• the account structure must be as described in the YP and here: db33786#diff-652796a822a40c46ab20f23cc3a62198R60

from pyethereum.

close in 216c50d

from pyethereum.

reopening since this commit does not handle key errors during deserialization.
returning the default value for the default key should solve this.

from pyethereum.

we just need to pass all the py.test.
but it's not related with this issue.

we can't revert to the earlier version,
because there are already serverl other fixes from various people.

from pyethereum.