shell: tweak export_value parameters.

Let’s have a default value for shell (considering this isn’t a public
API) to make it easier to use.

Library/Homebrew/cmd/--env.rb

@@ -37,7 +37,9 @@ module Homebrew
     if shell.nil?
       dump_build_env ENV
     else
-      env_keys.each { |key| puts Utils::Shell.export_value(shell, key, ENV[key]) }
+      env_keys.each do |key|
+        puts Utils::Shell.export_value(key, ENV[key], shell)
+      end
     end
   end
 end

Library/Homebrew/utils/shell.rb

@@ -21,7 +21,7 @@ module Utils
     end
 
     # quote values. quoting keys is overkill
-    def export_value(shell, key, value)
+    def export_value(key, value, shell = preferred)
       case shell
       when :bash, :ksh, :sh, :zsh
         "export #{key}=\"#{sh_quote(value)}\""