HTTPStatusCode enum is missing registered values

[djones6]

The public enum HTTPStatusCode is missing a number of values that are registered with IANA:
https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml

The ones that I spotted are:

• 423: Locked
• 425: Too Early
• 426: Upgrade Required

There may be others missing - it would be worth comparing against the IANA list.

Whilst adding cases to this public enum is technically a breaking change, we do not realistically expect someone to exhaustively switch over it (ie. without a default case), and so we feel this is a reasonable change to make as a bug fix.

This change needs to be made to both Kitura-net and Kitura-NIO.

[pushkarnk]

Looks like too early (425) doesn't exist in SwiftNIO's HTTPResponseStatus