Change default value of limit_posts from nil to 0 (see #1004)

Before this commit meaning of limit_posts:
  nil: no limit. generate all posts
  0: raise error
  n ( > 0): generate n posts only
  n ( < 0): raise error
  else: raise error

After this commit:
  nil: same as 0
  0: no limit. generate all posts
  n ( > 0): generate n posts only
  n ( < 0): raise error
  else: almost same as 0 (depend on `to_i` result)

lib/jekyll/configuration.rb

@@ -16,7 +16,7 @@ module Jekyll
 
       'safe'          => false,
       'show_drafts'   => nil,
-      'limit_posts'   => nil,
+      'limit_posts'   => 0,
       'lsi'           => false,
       'future'        => true,           # remove and make true just default
       'pygments'      => true,           # remove and make true just default

lib/jekyll/site.rb

@@ -28,7 +28,7 @@ module Jekyll
       self.future          = config['future']
       self.show_drafts     = config['show_drafts']
       # given as String if it is came from CLI option
-      self.limit_posts     = config['limit_posts'].nil? ? nil : config["limit_posts"].to_i
+      self.limit_posts     = config['limit_posts'].to_i
       self.keep_files      = config['keep_files']
 
       self.reset
@@ -63,8 +63,8 @@ module Jekyll
       self.categories      = Hash.new { |hash, key| hash[key] = [] }
       self.tags            = Hash.new { |hash, key| hash[key] = [] }
 
-      if !self.limit_posts.nil? && self.limit_posts < 1
-        raise ArgumentError, "Limit posts must be nil or >= 1"
+      if self.limit_posts < 0
+        raise ArgumentError, "Limit posts must be nil or >= 0"
       end
     end
 
@@ -149,7 +149,7 @@ module Jekyll
       self.posts.sort!
 
       # limit the posts if :limit_posts option is set
-      if limit_posts
+      unless limit_posts.zero?
         limit = self.posts.length < limit_posts ? self.posts.length : limit_posts
         self.posts = self.posts[-limit, limit]
       end

test/test_generated_site.rb

@@ -58,11 +58,11 @@ class TestGeneratedSite < Test::Unit::TestCase
       assert_equal 5, @site.posts.size
     end
 
-    should "ensure limit posts is 1 or more" do
+    should "ensure limit posts is 0 or more" do
       assert_raise ArgumentError do
         clear_dest
         stub(Jekyll).configuration do
-          Jekyll::Configuration::DEFAULTS.merge({'source' => source_dir, 'destination' => dest_dir, 'limit_posts' => 0})
+          Jekyll::Configuration::DEFAULTS.merge({'source' => source_dir, 'destination' => dest_dir, 'limit_posts' => -1})
         end
 
         @site = Site.new(Jekyll.configuration)