auto merge of #802 : alexcrichton/cargo/issue-801, r=brson

Previously all features were traversed when adding information to the lockfile,
but the traversal forgot to add optional dependencies that did not have a
corresponding feature.

Closes #801

diff --cc tests/test_cargo_features.rs
index 23569e4cdd8c3d7b109f534902e5eeaad28d959f,fa3d1111260c834a47ba98b9fd24072f4c716898..b5353f750ae323f044d9a804743869b5f5819032
--- 1/tests/test_cargo_features.rs
--- 2/tests/test_cargo_features.rs
+++ b/tests/test_cargo_features.rs
@@@ -1,5 -1,7 +1,7 @@@
+ use std::io::File;
+ 
  use support::{project, execs, cargo_dir};
 -use support::COMPILING;
 +use support::{COMPILING, FRESH};
  use support::paths::PathExt;
  use hamcrest::assert_that;