Add two new RFCs, remove one obsolete

doc/src/guide/specs.asciidoc

@@ -44,7 +44,6 @@ for or related to HTTP.
 * https://tools.ietf.org/html/rfc5789[RFC 5789]: PATCH Method for HTTP
 * https://tools.ietf.org/html/rfc5843[RFC 5843]: Additional Hash Algorithms for HTTP Instance Digests
 * https://tools.ietf.org/html/rfc5861[RFC 5861]: HTTP Cache-Control Extensions for Stale Content
-* https://tools.ietf.org/html/rfc5987[RFC 5987]: Character Set and Language Encoding for HTTP Header Field Parameters
 * https://tools.ietf.org/html/rfc5988[RFC 5988]: Web Linking
 * https://tools.ietf.org/html/rfc6265[RFC 6265]: HTTP State Management Mechanism
 * https://tools.ietf.org/html/rfc6266[RFC 6266]: Use of the Content-Disposition Header Field
@@ -83,7 +82,9 @@ for or related to HTTP.
 * https://tools.ietf.org/html/rfc7936[RFC 7936]: Clarifying Registry Procedures for the WebSocket Subprotocol Name Registry
 * https://tools.ietf.org/html/rfc8053[RFC 8053]: HTTP Authentication Extensions for Interactive Clients
 * https://tools.ietf.org/html/rfc8164[RFC 8164]: Opportunistic Security for HTTP/2
+* https://tools.ietf.org/html/rfc8187[RFC 8187]: Indicating Character Encoding and Language for HTTP Header Field Parameters
 * https://tools.ietf.org/html/rfc8188[RFC 8188]: Encrypted Content-Encoding for HTTP
+* https://tools.ietf.org/html/rfc8246[RFC 8246]: HTTP Immutable Responses
 * https://www.w3.org/TR/webmention/[Webmention]: Webmention
 
 ==== Upcoming