It was using net_version, but it should be using the eth_chainId method
introduced in EIP-695. I'm not sure whether/how the network ID differs
from the chain ID on mainnet and the testnets, but in Ganache in
particular, the network ID is 50 while the chain ID is 1337, and this
difference was causing problems for Python tests. Specifically, the
Web3.py interface `Web3.eth.chainId` invokes the eth_chainId method, and
the result feeds into the order hash, which wasn't lining up with the
non-Python side of things.
extending Mocha's ambient declarations.
`@0x/contracts-exchange`: Use `describe.optional` instead of
`blockchainTests.optional` in `test/fill_order.ts`.
