Merge pull request #583 from 0xProject/bug/contracts/erc721-encoded-pad

ERC721Proxy Pad to 32 if asset ids are small

packages/contracts/src/utils/asset_proxy_utils.ts

@@ -18,7 +18,8 @@ export const assetProxyUtils = {
     encodeUint256(value: BigNumber): Buffer {
         const formattedValue = new BN(value.toString(10));
         const encodedValue = ethUtil.toBuffer(formattedValue);
-        return encodedValue;
+        const paddedValue = ethUtil.setLengthLeft(encodedValue, 32);
+        return paddedValue;
     },
     encodeERC20ProxyData(tokenAddress: string): string {
         const encodedAssetProxyId = assetProxyUtils.encodeAssetProxyId(AssetProxyId.ERC20);