zeroex: remove checksum from address fields in Order when marshalling to JSON

[hrharder]

Overview

Simply adding a call to strings.ToLower() on all address fields when marshalling a SignedOrder to JSON. Tests (run with make test-all) pass and functionality seems normal. Please advise if a different approach is preferred.

Should close #257.

Notes

Here is the output I got from a mesh_subscription message after adding a valid order (compare to sample in original issue).

{
	"jsonrpc": "2.0",
	"method": "mesh_subscription",
	"params": {
		"subscription": "0x5375e6c2d24cb11874a79ea90e7851f",
		"result": [{
			"fillableTakerAssetAmount": "0",
			"kind": "EXPIRED",
			"orderHash": "0x222f59940699cb7f7743d68e95cc952eabef58dd3ef22629811f0da6ff8b29aa",
			"signedOrder": {
				"makerAddress": "0x5409ed021d9299bf6814279a6a1411a7e866a631",
				"makerAssetData": "0xf47261b000000000000000000000000034d402f14d58e001d8efbe6585051bf9706aa064",
				"makerAssetAmount": "1000000000000000000",
				"makerFee": "0",
				"takerAddress": "0x0000000000000000000000000000000000000000",
				"takerAssetData": "0xf47261b000000000000000000000000025b8fe1de9daf8ba351890744ff28cf7dfa8f5e3",
				"takerAssetAmount": "98000000000000000000",
				"takerFee": "0",
				"senderAddress": "0x0000000000000000000000000000000000000000",
				"exchangeAddress": "0x48bacb9266a570d521063ef5dd96e61686dbe788",
				"feeRecipientAddress": "0x0000000000000000000000000000000000000000",
				"expirationTimeSeconds": "1563239813",
				"salt": "50548472960658633474509830344688430210810445656575157876862263167070060655407",
				"signature": "0x1c197ae36570bec706392e00594be1127caafe36f4a8093ed558039056cbb709553101ac01526e768d1d6c575cc7bf65dca4e140c27cb52228f2fba6278e19426a02"
			},
			"txHashes": []
		}]
	}
}

Let me know if think this should be done somewhere else, differently, etc... Or if someone else has already tackled it!

[fabioberger]

@hrharder the MakerAssetAmount should be a numerical number and therefore does not need to be lowercased.

[hrharder]

Ahh yes -- thank you 🤦‍♂

[fabioberger]

Suggested change

	MakerAddress: strings.ToLower(s.MakerAddress.String()),
	MakerAddress: strings.ToLower(s.MakerAddress.Hex()),

Nit: String() simply points to Hex() under-the-hood and exists as an implementation of the stringer interface. I feel it's slightly more expressive to use Hex() here. Mind making this change in the other places too?

[hrharder]

Of course, I totally agree. I tried .String() (before checking implementation) to see if go-ethereum offered a native non-checksummed stringer, and never reverted.