Auto merge of #13058 - malisas:malisa-responseAPI, r=Manishearth,jdm

Response API

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This PR adds the [dom::Response](https://fetch.spec.whatwg.org/#response-class) implementation and addresses #11896.

The relevant passing tests` expectations have been updated.

In order to allow non-UTF-8-encoded status messages, `net_traits::response::Response`'s `raw_status` field has been changed from type [`Option<RawStatus>`](https://doc.servo.org/hyper/http/struct.RawStatus.html) to type `Option<(u16, Vec<u8>)>`. As a result, a few other files which rely on the `raw_status` field were affected and updated.

TODOs:
- The `body` and `trailer` methods. Relies on implementation of `ReadableStream` and `Promise`s.
- Similarly, replace the dummy constructor `_body: Option<USVString>` argument with `body: ResponseBodyInit`.
- Currently, whenever `r's response's header list` or `r's Headers object` are mentioned, I always modify the `headers_reflector` field (of type dom::Headers, or `r's Headers object`) and not the corresponding hyper::Headers list in the net_traits::Response field. A completely accurate interpretation of the spec might consider making both of these lists the same thing via a reference. [Discussion](https://github.com/whatwg/fetch/issues/358) was [had](https://github.com/servo/servo/pull/12884).

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- [ ] These changes fix #__ (github issue number if applicable).

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components/servo/Cargo.lock

@@ -493,6 +493,7 @@ name = "devtools"
 version = "0.0.1"
 dependencies = [
  "devtools_traits 0.0.1",
+ "encoding 0.2.32 (registry+https://github.com/rust-lang/crates.io-index)",
  "hyper 0.9.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "hyper_serde 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "ipc-channel 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",