Save reason phrase in Protocol::HTTP1::Response

Save away the raw reason phrase provided by the server. This is of
limited niche value, as the reason phrase seems to be _mostly_
unused/ignored.

One example of where it's needed:

The [Proxmox VE API](https://pve.proxmox.com/wiki/Proxmox_VE_API)
ships error reason strings back using the HTTP reason phrase. Without
this side-channel, error status is 100% opaque/unavailable.

lib/async/http/protocol/http1/response.rb

@@ -18,9 +18,10 @@ def self.read(connection, request)
 
 					UPGRADE = 'upgrade'
 
-					# @param reason [String] HTTP response line reason, ignored.
+					# @param reason [String] HTTP response line reason phrase
 					def initialize(connection, version, status, reason, headers, body)
 						@connection = connection
+						@reason = reason
 
 						protocol = headers.delete(UPGRADE)
 
@@ -30,6 +31,10 @@ def initialize(connection, version, status, reason, headers, body)
 					def connection
 						@connection
 					end
+
+					def reason
+						@reason
+					end
 
 					def hijack?
 						@body.nil?