fix tests

test/test_site.rb

@@ -124,7 +124,7 @@ class TestSite < Test::Unit::TestCase
     end
 
     should "read posts" do
-      @site.read_posts('')
+      @site.read_posts('', '_posts')
       posts = Dir[source_dir('_posts', '*')]
       assert_equal posts.size - 1, @site.posts.size
     end

test/test_tags.rb

@@ -11,7 +11,7 @@ class TestTags < Test::Unit::TestCase
     site = Site.new(Jekyll.configuration)
 
     if override['read_posts']
-      site.read_posts('')
+      site.read_posts('', '_posts')
     end
 
     info = { :filters => [Jekyll::Filters], :registers => { :site => site } }