Add RFC7233 support (#299)

Add RFC7233 support a.k.a "Range" headers support

src/kemal/response.cr

@@ -0,0 +1,14 @@
+class HTTP::Server::Response
+  class Output
+    def close
+
+      unless response.wrote_headers? && !response.headers.has_key?("Content-Range")
+        response.content_length = @out_count
+      end
+
+      ensure_headers_written
+
+      super
+    end
+  end
+end

src/kemal/static_file_handler.cr

@@ -56,6 +56,9 @@ module Kemal
         request_headers = context.request.headers
         filesize = File.size(file_path)
         File.open(file_path) do |file|
+          if context.request.headers.has_key?("Range") && context.request.method == "GET"
+            next multipart(file, context)
+          end
           if request_headers.includes_word?("Accept-Encoding", "gzip") && config.is_a?(Hash) && config["gzip"] == true && filesize > minsize && Utils.zip_types(file_path)
             context.response.headers["Content-Encoding"] = "gzip"
             Zlib::Deflate.gzip(context.response) do |deflate|
@@ -76,6 +79,58 @@ module Kemal
       end
     end
 
+    def multipart(file, env)
+      #See http://httpwg.org/specs/rfc7233.html
+      fileb = file.size
+
+      range = env.request.headers["Range"]
+      match = range.match(/bytes=(\d{1,})-(\d{0,})/)
+
+      startb = 0
+      endb = 0
+
+      if match
+        if match.size >= 2
+          startb = match[1].to_i { 0 }
+        end
+
+        if match.size >= 3
+          endb = match[2].to_i { 0 }
+        end
+      end
+
+      if endb == 0
+          endb = fileb
+      end
+
+      if startb < endb && endb <= fileb
+        env.response.status_code = 206
+        env.response.content_length = endb - startb
+        env.response.headers["Accept-Ranges"] = "bytes"
+        env.response.headers["Content-Range"] = "bytes #{startb}-#{endb - 1}/#{fileb}" # MUST
+
+        if startb > 1024
+          skipped = 0
+          # file.skip only accepts values less or equal to 1024 (buffer size, undocumented)
+          until skipped + 1024 > startb
+            file.skip(1024)
+            skipped += 1024
+          end
+          if skipped - startb > 0
+            file.skip(skipped - startb)
+          end
+        else
+          file.skip(startb)
+        end
+
+        IO.copy(file, env.response, endb - startb)
+      else
+        env.response.content_length = fileb
+        env.response.status_code = 200 # Range not satisfable, see 4.4 Note
+        IO.copy(file, env.response)
+      end
+    end
+
     def etag(context, file_path)
       etag = %{W/"#{File.lstat(file_path).mtime.epoch.to_s}"}
       context.response.headers["ETag"] = etag