Allow ethclient to fetch blocks with empty uncles (#228)

The first blocks of the celo chain (till gingerbread) lacked the
uncles field, so to support retrieving them with the
ethclient.Client this modification is required.

ethclient/ethclient.go

@@ -150,7 +150,10 @@ func (ec *Client) getBlock(ctx context.Context, method string, args ...interface
  if head.UncleHash == types.EmptyUncleHash && len(body.UncleHashes) > 0 {
  return nil, errors.New("server returned non-empty uncle list but block header indicates no uncles")
  }
- if head.UncleHash != types.EmptyUncleHash && len(body.UncleHashes) == 0 {
+ // In celo before the ginerbread hardfork, blocks had no uncles hash and no
+ // uncles, so in those cases it is legitimate to have an empty uncles hash.
+ var emptyHash common.Hash
+ if head.UncleHash != emptyHash && head.UncleHash != types.EmptyUncleHash && len(body.UncleHashes) == 0 {
  return nil, errors.New("server returned empty uncle list but block header indicates uncles")
  }
  if head.TxHash == types.EmptyTxsHash && len(body.Transactions) > 0 {