core: content-type is case insensitive for metadata check [GH-4369]

CHANGELOG.md

@@ -46,6 +46,8 @@ BUG FIXES:
       as root. [GH-4387]
   - core: "vagrant provision" will cause "vagrant up" to properly not
       reprovision. [GH-4393]
+  - commands/box/add: "Content-Type" header is now case-insensitive when
+      looking for metadata type. [GH-4369]
   - commands/package: base package won't crash with exception [GH-4017]
   - commands/rsync-auto: Destroyed machines won't raise exceptions. [GH-4031]
   - commands/ssh: Extra args are passed through to Docker container. [GH-4378]

lib/vagrant/action/builtin/box_add.rb

@@ -481,7 +481,7 @@ module Vagrant
           end
 
           output = d.head
-          match  = output.scan(/^Content-Type: (.+?)$/).last
+          match  = output.scan(/^Content-Type: (.+?)$/i).last
           return false if !match
           match.last.chomp == "application/json"
         end