Fix JSON serialization of Env.block.transaction

[webmaster128]

The JSON field name annotation in this line is wrong:

wasmvm/types/env.go

Line 13 in 1afba37

Transaction *TransactionInfo `json:"transaction_info,omitempty"`

. It needs to be called transaction. Right now the transaction field is undefined, deserializing to None in the contract. The extra transaction_info field is ignored in deserialization.

This bug was introduced in #260, i.e. never worked before.