Auto merge of #13345 - malisas:malisa-bodyAPI, r=jdm

implement Body for dom::Response and dom::Request

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This PR adds dom::Response and dom::Request's `Text()`, `Blob()`, `Json()`, and (part of the) `FormData()` functions, part of the [Body mixin](https://fetch.spec.whatwg.org/#body). The corresponding tests have also been updated as passing.

@jeenalee also contributed to this PR, primarily the `Blob()`, `Json()`, and `FormData()` implementations.

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

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components/script/lib.rs

@@ -92,6 +92,7 @@ extern crate websocket;
 extern crate xml5ever;
 
 pub mod bluetooth_blacklist;
+mod body;
 pub mod clipboard_provider;
 mod devtools;
 pub mod document_loader;