Unit test for util::str::split_html_space_chars

I was concerned for a little bit that passing just whitespace to the
split_html_space_chars function would result in len > 0, so I wrote this
small unit test to make sure it does the right thing. Even though it
does do the right thing, I think there's still value in committing the
unit test.

tests/unit/util/lib.rs

@@ -13,3 +13,4 @@ extern crate euclid;
 #[cfg(test)] mod task;
 #[cfg(test)] mod vec;
 #[cfg(test)] mod mem;
+#[cfg(test)] mod str;