core: don't make all downloads continuable

lib/vagrant/action/builtin/box_add.rb

@@ -29,6 +29,7 @@ module Vagrant
           end
 
           downloader_options = {}
+          downloader_options[:continue] = true
           downloader_options[:insecure] = env[:box_download_insecure]
           downloader_options[:ui] = env[:ui]
 

lib/vagrant/util/downloader.rb

@@ -20,6 +20,7 @@ module Vagrant
 
         # Get the various optional values
         options     ||= {}
+        @continue    = options[:continue]
         @insecure    = options[:insecure]
         @ui          = options[:ui]
       end
@@ -38,9 +39,9 @@ module Vagrant
           "--max-redirs", "10",
           "--user-agent", USER_AGENT,
           "--output", @destination,
-          "--continue-at", "-"
         ]
 
+        options += ["--continue-at", "-"] if @continue
         options << "--insecure" if @insecure
         options << @source