core: download metadata with JSON accept header

lib/vagrant/action/builtin/box_add.rb

@@ -134,7 +134,7 @@ module Vagrant
 
           metadata = nil
           begin
-            metadata_path = download(url, env, ui: false)
+            metadata_path = download(url, env, json: true, ui: false)
 
             File.open(metadata_path) do |f|
               metadata = BoxMetadata.new(f)
@@ -391,6 +391,7 @@ module Vagrant
           downloader_options[:continue] = true
           downloader_options[:insecure] = env[:box_download_insecure]
           downloader_options[:client_cert] = env[:box_client_cert]
+          downloader_options[:headers] = ["Accept: application/json"] if opts[:json]
           downloader_options[:ui] = env[:ui] if opts[:ui]
 
           Util::Downloader.new(url, temp_path, downloader_options)

lib/vagrant/util/downloader.rb

@@ -26,6 +26,7 @@ module Vagrant
         options     ||= {}
         @ca_cert     = options[:ca_cert]
         @continue    = options[:continue]
+        @headers     = options[:headers]
         @insecure    = options[:insecure]
         @ui          = options[:ui]
         @client_cert = options[:client_cert]
@@ -180,6 +181,12 @@ module Vagrant
         options << "--insecure" if @insecure
         options << "--cert" << @client_cert if @client_cert
 
+        if @headers
+          Array(@headers).each do |header|
+            options << "-H" << header
+          end
+        end
+
         # Specify some options for the subprocess
         subprocess_options = {}