add HTTPHeaders.first(name:)

[MahdiBM]

Adds a new method first(name:) to HTTPHeaders (aka just [String: String]).

Motivation:

I first noticed absence of such a function when I was trying to get 2 different headers of a Github request.
Using capitalized header names as mentioned in Github docs, or lowercased names, both would fail.
(EDIT: To be clear, i think one header had a capitalized name and the other a lowercased name, which is just an inconsistency by Github in this case.)
That was when I copy-pasted swift-nio's HTTPHeaders.first(name:) function to the lambda target, which fixed the issue.

I think this will be a generally useful function, as it's even available in NIO's implementation of HTTPHeaders.

Modifications:

Copy-paste NIO's implementation of HTTPHeaders.first(name:).
Modify the comments to be correct about this project.
Declare the extension on String.UTF8View instead of Sequence<UInt8> as we don't need the generic extension unlike NIO.

Update: Copy-pasted the tests as well.

Result:

Addition of a function to the public API HTTPHeaders.first(name:) (aka Dictionary<String, String>.first(name:)).

[tomerd]

@swift-server-bot test this please

[tomerd]

thanks @MahdiBM

[MahdiBM]

@tomerd another try please 🙂

[tomerd]

@swift-server-bot test this please

[MahdiBM]

Hmm correct me if I'm wrong but the failure seems unrelated to the PR. Possibly just needs another retry.

[tomerd]

@swift-server-bot test this please

[tomerd]

thanks @MahdiBM