HeaderMap::iter could be ExactSizedIterator

[anuraaga]

I was surprised that iterating over the entries in the HeaderMap doesn't provide an exact size. This is mainly because there is a len() method with the size of the map. I can provide an ExactSizedIterator wrapper but was wondering if it makes sense to implement it here to make it easier to preallocate transformations?

[seanmonstar]

Yea it probably could be! While HeaderMap doesn't allocate linearly because of it being a multi-map, it does know exactly how many values are in the map.