Fix for String#undent

Without it, String#undent would fail on unindented strings, e.g.:

    "foo".undent

NoMethodError: undefined method `length' for nil:NilClass`

Closes Homebrew/homebrew#28873.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/extend/string.rb

@@ -1,6 +1,6 @@
 class String
   def undent
-    gsub(/^.{#{slice(/^ +/).length}}/, '')
+    gsub(/^.{#{(slice(/^ +/) || '').length}}/, '')
   end
 
   # eg:

Library/Homebrew/test/test_string.rb

@@ -10,4 +10,12 @@ class StringTest < Test::Unit::TestCase
     EOS
     assert undented == "hi\nmy friend over\nthere\n"
   end
+
+  def test_undent_not_indented
+    undented = <<-EOS.undent
+hi
+I'm not indented
+    EOS
+    assert undented == "hi\nI'm not indented\n"
+  end
 end