Update HTTP Status Codes

Updates status codes in `Rack::Utils.HTTP_STATUS_CODES` to [RFC 7231](https://tools.ietf.org/html/rfc7231).
Source: [csv](https://www.iana.org/assignments/http-status-codes/http-status-codes-1.csv) from [IANA HTTP Status Code Registry](https://www.iana.org/assignments/http-status-codes), updated 2014-08-27

- **306:** [reserved, no longer used](https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-26#section-6.4.6)
- **413:** [changed reason-phrase - ~~Request Entity~~ _**Payload** Too Large_](https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-26#section-6.5.11)
- **414:** [changed reason-phrase - ~~Request-~~_URI Too Long_](https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-26#section-6.5.12)
- **416:** [changed reason-phrase - ~~Requested~~ _Range Not Satisfiable_](https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-26#section-4.4)
- **418:** I'm no longer a teapot :tea: 
- **506:** changed reason-phrase - _Variant Also Negotiates_ ~~(Experimental)~~

Updates comment about code generation.