PriceOracle return constant price when base asset and quote asset are same

[gzliudan]

In PriceOracle, when base asset equals quote asset, we can return 10**18 directly, no need to run other rules.

    function getPrice(address _assetOne, address _assetTwo) external view returns (uint256) {
        require(
            controller.isSystemContract(msg.sender),
            "PriceOracle.getPrice: Caller must be system contract."
        );

        if (_assetOne == _assetTwo) {
            return 10**18;
        }