add support for 100-continue in HTTP::Server::Response

spec/std/http/server/server_spec.cr

@@ -216,6 +216,15 @@ module HTTP
         response.respond_with_error("Bad Request", 400)
         io.to_s.should eq("HTTP/1.1 400 Bad Request\r\nContent-Type: text/plain\r\nTransfer-Encoding: chunked\r\n\r\n10\r\n400 Bad Request\n\r\n")
       end
+
+      it "supports 100-continue" do
+        io = IO::Memory.new
+        response = Response.new(io)
+        response.continue
+        response.content_type = "text/html"
+        response << "test"
+        io.to_s.should eq("HTTP/1.1 100 Continue\r\n\r\nHTTP/1.1 200\r\n\r\ntest\r\n")
+      end
     end
   end
 

src/http/server/response.cr

@@ -79,6 +79,15 @@ class HTTP::Server
       raise "Can't read from HTTP::Server::Response"
     end
 
+    # Send a 100 Continue response to clients waiting for this before sending
+    # the request body
+    def continue
+      @status_code = 100
+      write_headers
+      flush
+      reset
+    end
+
     # Upgrades this response, writing headers and yieling the connection `IO` (a socket) to the given block.
     # The block must invoke `close` afterwards, the server won't do it in this case.
     # This is useful to implement protocol upgrades, such as websockets.