Update example in Http::Headers object to use .from_hash

[javierjulio]

Summary

The documented example at the top of ActionDispatch::Http::Headers definition provided a hash for the constructor but in Rails 5 this has changed to now take a request object. An earlier commit introduced a ActionDispatch::Http::Headers.from_hash() method that the example should now be using.

Other Information

When initializing an ActionDispatch::Http::Headers object it takes a request object (Rails 5) whereas before it took a hash (Rails 4.x) but the documented example still shows a hash given to the constructor (due to commit 34fa665 by @tenderlove) so this is just a documentation change to use the new from_hash method introduced in that earlier commit.

This is my first PR (I've reported an issue or two before) so I might have the process wrong. Sorry, I was confused with the guidelines on doc changes since this is for the documented example in the code file rather than pages in the guides. I take it its done in the right place. If not let me know what I have to do and be happy to update. Thanks.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @schneems (or someone else) soon.

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[maclover7]

Great first PR @javierjulio!! This looks good to me, but in the future, please add [ci skip] to the body of your Git commit to avoid running Travis CI for documentation changes.

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