More allocator friendly Header definitions

[arthurprs]

I was looking at TechEmpower/FrameworkBenchmarks#2611 the other day and I noticed there's no way of using some Header types without allocating, like the Server header in the linked example.

It's not a problem per se but it's probably worth discussing as setting N headers could potentially allocate N times.

[seanmonstar]

Agreed. I starting with the most common, Host, but others should follow. Since changing the signature would be a breaking change anyways, it also makes sense to make the fields private and use constructors instead. That way we can start with Cow but we can switch to Bytes or something later in the future and not break. Some common headers that could benefit:

• Location
• Origin
• Referer
• Server
• UserAgent

[mcarlin]

@seanmonstar, I'm looking to start contributing here. Mind if I take a stab at it and convert over those headers you mentioned if you're not already working on it?

[seanmonstar]

@mcarlin That'd be great! I turned the list into checkboxes, ideally we can keep them up to date.

[seanmonstar]

The list of headers I made have all been converted. If there's changes want to a specific header, lets open specific issues. Thanks!