Unwrap WHBAR for HBAR

/// @notice Safely unwrap whbar to msg.sender
/// @dev This contract needs an allowance from msg.sender to transfer the whbar token
/// @param wad The amount to unwrap
function unwrapWhbar(uint wad) public {
  require(wad > 0, "WhbarHelper: wad cannot be lt zero");
  // transfer the whbar to this contract
  HederaTokenHelper.safeTransferFrom(whbarToken, msg.sender, address(this), wad);
  
  // approve sending the whbar to the whbar contract
  HederaTokenHelper.safeApprove(whbarToken, whbarContract, wad);
  
  // use withdraw(address src, address dst, uint wad) and use this contract and msg.sender to withdraw
  // to the contract caller
  IWHBAR(whbarContract).withdraw(address(this), msg.sender, wad);
}