Rename HTTP.rfc1123_date to HTTP.format_date

The date format used for HTTP applications (such as cookies) according to [RFC 2612](https://tools.ietf.org/html/rfc2616#section-3.3.1)
is *not* an arbitrary [RFC 1123](https://tools.ietf.org/html/rfc1123#page-55) string but has a fixed timezone `GMT`. Therefore it
is not the same as `Time#to_rfc2822` and the method should not be called `rfc1123_date`.

spec/std/http/http_spec.cr

@@ -37,10 +37,10 @@ describe HTTP do
     parsed_time.to_utc.to_s.should eq("2011-09-10 02:36:00 UTC")
   end
 
-  describe "generates RFC 1123" do
+  describe "generates HTTP date" do
     it "without time zone" do
       time = Time.new(1994, 11, 6, 8, 49, 37, 0, Time::Kind::Utc)
-      HTTP.rfc1123_date(time).should eq("Sun, 06 Nov 1994 08:49:37 GMT")
+      HTTP.format_date(time).should eq("Sun, 06 Nov 1994 08:49:37 GMT")
     end
 
     it "with local time zone" do
@@ -49,7 +49,7 @@ describe HTTP do
       LibC.tzset
       begin
         time = Time.new(1994, 11, 6, 8, 49, 37, 0, Time::Kind::Local)
-        HTTP.rfc1123_date(time).should eq(time.to_utc.to_s("%a, %d %b %Y %H:%M:%S GMT"))
+        HTTP.format_date(time).should eq(time.to_utc.to_s("%a, %d %b %Y %H:%M:%S GMT"))
       ensure
         ENV["TZ"] = tz
         LibC.tzset

src/http/common.cr

@@ -228,14 +228,22 @@ module HTTP
     nil
   end
 
-  # Format a Time object as a String using the format specified by [RFC 1123](https://tools.ietf.org/html/rfc1123#page-55).
+  # Format a `Time` object as a `String` using the format specified as `sane-cookie-date`
+  # by [RFC 6265](https://tools.ietf.org/html/rfc6265#section-4.1.1) which is
+  # according to [RFC 2616](https://tools.ietf.org/html/rfc2616#section-3.3.1) a
+  # [RFC 1123](https://tools.ietf.org/html/rfc1123#page-55) format with explicit
+  # timezone `GMT` (interpreted as `UTC`).
   #
   # ```
-  # HTTP.rfc1123_date(Time.new(2016, 2, 15)) # => "Sun, 14 Feb 2016 21:00:00 GMT"
+  # HTTP.format_http_date(Time.new(2016, 2, 15)) # => "Sun, 14 Feb 2016 21:00:00 GMT"
   # ```
-  # DEPRECATED: Use `Time#to_rfc2822` instead.
+  def self.format_date(time : Time) : String
+    time.to_utc.to_s("%a, %d %b %Y %H:%M:%S GMT")
+  end
+
+  # DEPRECATED: Use `HTTP.format_http_date` instead.
   def self.rfc1123_date(time : Time) : String
-    time.to_rfc2822
+    format_date(time)
   end
 
   # Dequotes an [RFC 2616](https://tools.ietf.org/html/rfc2616#page-17)

src/http/cookie.cr

@@ -31,7 +31,7 @@ module HTTP
         header << "#{URI.escape @name}=#{URI.escape value}"
         header << "; domain=#{domain}" if domain
         header << "; path=#{path}" if path
-        header << "; expires=#{expires.to_utc.to_rfc2822}" if expires
+        header << "; expires=#{HTTP.format_date(expires)}" if expires
         header << "; Secure" if @secure
         header << "; HttpOnly" if @http_only
         header << "; #{@extension}" if @extension