Context: #70 #71

As setting deferSend implies keepalive is also true, such request has to share the same size limit budget as a regular keepalive request’s one: "for each fetch group, the sum of contentLength and inflightKeepaliveBytes <= 64 KB".

To comply with the limit, there are several options:

1. fetch() throws TypeError whenever the budget has exceeded. Users will not be able to create new pending requests.
2. The browser forces sending out other existing pending requests, in FIFO order, when the budget has exceeded. For a single request > 64KB, fetch() should still throws TypeError.
3. Ignore the size limit if BackgroundFetch Permission is enabled for the page.