Panic due to the max headers capacity

[DDtKey]

This method https://github.com/hyperium/h2/blob/6a75f232330374d5f329aaae91afc2dee7ed2b1f/src/frame/headers.rs#L830-L922 panics if the number of headers is too high

It's directly related to https://github.com/hyperium/http/issues/603

[seanmonstar]

I notice there is a guard in there that if the size is over max_header_list_size, it won't get appended to the HeaderMap. Is what you've noticed when there's a bunch of tiny headers?