Improved permalinks for pages and collections

This updates the default permalink style for pages and collections to
match the site-wide 'permalink' setting.  If the permalink setting
contains a trailing slash, either explicitly or by being set to
':pretty', then pages and collections permalinks will contain trailing
slashes by default as well.  Similarly, if the permalink setting
contains a trailing ':output_ext', so will pages and collections.  If
the permalink setting contains neither a trailing slash or extension,
neither will pages or collections.

This impacts only the default permalink structure for pages and
collections.  Permalinks set in the frontmatter of an individual page
take precedence, as does the permalink setting for a specific
collection.

Fixes #2691

test/test_document.rb

@@ -219,7 +219,31 @@ class TestDocument < JekyllUnitTest
       assert_equal "/slides/test/example-slide-1", @document.url
     end
 
-    should "produce the right destination" do
+    should "produce the right destination file" do
+      assert_equal @dest_file, @document.destination(dest_dir)
+    end
+  end
+
+  context "a document in a collection with pretty permalink style" do
+    setup do
+      @site = fixture_site({
+        "collections" => {
+          "slides" => {
+            "output"    => true,
+          }
+        },
+      })
+      @site.permalink_style = :pretty
+      @site.process
+      @document = @site.collections["slides"].docs[0]
+      @dest_file = dest_dir("slides/example-slide-1/index.html")
+    end
+
+    should "produce the right URL" do
+      assert_equal "/slides/example-slide-1/", @document.url
+    end
+
+    should "produce the right destination file" do
       assert_equal @dest_file, @document.destination(dest_dir)
     end
   end