fix: when extending make sure user does not pay for time in the past

Signed-off-by: Tomás Migone <tomas@edgeandnode.com>

contracts/contracts/Subscriptions.sol

@@ -268,7 +268,7 @@ contract Subscriptions is Ownable {
     /// @notice Extends a subscription's end time.
     /// The time the subscription will be extended by is calculated as `amount / rate`, where
     /// `rate` is the existing subscription rate and `amount` is the new amount of tokens provided.
-    /// The new end time must be in the future.
+    /// If the subscription was expired the extension will start from the current block timestamp.
     /// @dev The function's name, `addTo`, is used to comply with the `IPayment` interface for recurring payments.
     /// @param user Subscription owner.
     /// @param amount Total amount to be added to the subscription.
@@ -280,7 +280,7 @@ contract Subscriptions is Ownable {
         require(sub.start != 0, 'no subscription found');
         require(sub.rate != 0, 'cannot extend a zero rate subscription');
 
-        uint64 oldEnd = sub.end;
+        uint64 oldEnd = uint64(Math.max(sub.end, block.timestamp));
         uint64 newEnd = oldEnd + uint64(amount / sub.rate);
         require(block.timestamp < newEnd, 'new end cannot be in the past');