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/str.rs

@@ -0,0 +1,12 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use util::str::split_html_space_chars;
+
+
+#[test]
+pub fn split_html_space_chars_whitespace() {
+    assert!(split_html_space_chars("").collect::<Vec<_>>().is_empty());
+    assert!(split_html_space_chars("\u{0020}\u{0009}\u{000a}\u{000c}\u{000d}").collect::<Vec<_>>().is_empty());
+}