Prevent panic when calling url::Url::set_host with None.

Fixes #243.

src/lib.rs

@@ -1003,7 +1003,7 @@ impl Url {
             debug_assert!(self.byte_at(self.scheme_end) == b':');
             debug_assert!(self.byte_at(self.path_start) == b'/');
             let new_path_start = self.scheme_end + 1;
-            self.serialization.drain(self.path_start as usize..new_path_start as usize);
+            self.serialization.drain(new_path_start as usize..self.path_start as usize);
             let offset = self.path_start - new_path_start;
             self.path_start = new_path_start;
             self.username_end = new_path_start;

tests/unit.rs

@@ -317,3 +317,12 @@ fn append_empty_segment_then_mutate() {
     url.assert_invariants();
     assert_eq!(url.to_string(), "http://localhost:6767/foo/bar?a=b");
 }
+
+#[test]
+fn test_set_host() {
+    let mut url = Url::parse("https://example.net/hello").unwrap();
+    url.set_host(Some("foo.com")).unwrap();
+    assert_eq!(url.as_str(), "https://foo.com/hello");
+    url.set_host(None).unwrap();
+    assert_eq!(url.as_str(), "https:/hello");
+}