Fixed a bug where all listings had eth as the currency.

Before

All listings, even USDC and other currencies, were reported as ETH.

 {
      "order_hash": "0x9f06c8646fa49ca41ec4686dfc3f8c3e4d8c99b63ba67056a7c23a8673a62947",
      "chain": "ethereum",
      "type": "basic",
      "price": {
        "current": {
          "currency": "eth",
          "value": "3000000000"
        }
      },
      "protocol_data": {
        ...
    }

After

Listings will report the correct currency in the price field.

 {
      "order_hash": "0x9f06c8646fa49ca41ec4686dfc3f8c3e4d8c99b63ba67056a7c23a8673a62947",
      "chain": "ethereum",
      "type": "basic",
      "price": {
        "current": {
          "currency": "USDC",
          "value": "3000000000"
        }
      },
      "protocol_data": {
        ...
    }