[Haml] Interpret #{} at the beginning of a line as interpolation.

Closes gh-1

lib/haml/precompiler.rb

@@ -195,7 +195,10 @@ END
       @index = index + 1
 
       case text[0]
-      when DIV_CLASS, DIV_ID; render_div(text)
+      when DIV_CLASS; render_div(text)
+      when DIV_ID
+        return push_plain(text) if text[1] == ?{
+        render_div(text)
       when ELEMENT; render_tag(text)
       when COMMENT; render_comment(text[1..-1].strip)
       when SANITIZE

test/haml/engine_test.rb

@@ -139,6 +139,11 @@ class EngineTest < Test::Unit::TestCase
                  render("\"title '\#{\"Title\"}'. \""))
   end
 
+  def test_interpolation_at_the_beginning_of_a_line
+    assert_equal("<p>2</p>\n", render('%p #{1 + 1}'))
+    assert_equal("<p>\n  2\n</p>\n", render("%p\n  \#{1 + 1}"))
+  end
+
   def test_nil_tag_value_should_render_as_empty
     assert_equal("<p></p>\n", render("%p= nil"))
   end