core: do proper type conversions for box checksum types

lib/vagrant/action/builtin/box_add.rb

@@ -19,8 +19,8 @@ module Vagrant
           @download_interrupted = false
 
           # Determine the checksum type to use
-          checksum = env[:box_checksum] || ""
-          checksum_klass = case env[:box_checksum_type]
+          checksum = (env[:box_checksum] || "").to_s
+          checksum_klass = case env[:box_checksum_type].to_sym
           when nil
             nil
           when :md5

plugins/commands/box/command/add.rb

@@ -16,7 +16,7 @@ module VagrantPlugins
             end
 
             o.on("--checksum-type VALUE", String, "Checksum type") do |c|
-              options[:checksum_type] = c
+              options[:checksum_type] = c.to_sym
             end
 
             o.on("-c", "--clean", "Remove old temporary download if it exists.") do |c|

plugins/kernel_v2/config/vm.rb

@@ -270,6 +270,10 @@ module VagrantPlugins
         @hostname = nil if @hostname == UNSET_VALUE
         @hostname = @hostname.to_s if @hostname
 
+        if @box_download_checksum_type
+          @box_download_checksum_type = @box_download_checksum_type.to_sym
+        end
+
         # Make sure the box URL is an array if it is set
         if @box_url && !@box_url.is_a?(Array)
           @box_url = [@box_url]