Fix step 31 of the Request constructor

[nox]

Step 31 says:

Fill r's Headers object with headers. Rethrow any exceptions.

But the code just does:

r.Headers().fill(Some(HeadersOrByteStringSequenceSequence::Headers(headers_copy)));

The compiler even rightly complains:

warning: unused result which must be used, #[warn(unused_must_use)] on by default
   --> /Users/nox/src/servo/components/script/dom/request.rs:342:9
    |
342 |         r.Headers().fill(Some(HeadersOrByteStringSequenceSequence::Headers(headers_copy)));
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The code for step 31 should be wrapped in try!().

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[nox]

Cc @jeenalee.

[jeenalee]

I'd like to work on this. Thank you for filing this issue!

[nox]

Sure. :)