feat(headers): Add Content-Disposition header

[mikedilger]

fixes #561

This is fairly extensive and would benefit from some review.
It does not try to interpret language-tags (rfc 5646) but rather accepts any string (for now).

[seanmonstar]

This should be sections.splitn(2, '='), in case any values are foo=bar=baz.

[seanmonstar]

@mikedilger indeed, this is a lot of work! awesome!

I've reviewed the code, though I must say, I'm not too familiar with this header, so I may have missed something.

[mikedilger]

I think I have addressed everything above except using url::percent_encoding, which depends on PR servo/rust-url#134

[seanmonstar]

Seems a new version is needed of the url crate?

[mikedilger]

Precisely. I'm in no hurry to get this PR merged as I can use raw headers in the meantime. But if you like things tidy, @SimonSapin would have to make a release.

[SimonSapin]

I just published url 0.3.0 0.2.38 on crates.io which includes servo/rust-url#134. Publishing is easy but I sometimes forget to do it. Feel free to ask for it in PRs!

[seanmonstar]

@mikedilger I see it's passing now. Awesome!

Could you rebase this into a single commit, feat(headers): add Content-Disposition ...? The current commits will make the changelog confusing (fixing bugs that were never released, etc).

[mikedilger]

Ok, rebased onto master and collapsed into a single commit

[seanmonstar]

Seems another PR updated the Cargo.toml to bump the url version already, making a conflict here.

[mikedilger]

Ok, rebased again.

[seanmonstar]

\o/