[whatwg/xhr] Use XMLHttpRequestBodyInit as defined in Fetch, in send. (#279)

Following-up on https://github.com/whatwg/fetch/pull/1029


FIX #277



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### :boom: Error: 400 Bad Request :boom: ###

[PR Preview](https://github.com/tobie/pr-preview#pr-preview) failed to build. _(Last tried on May 31, 2020, 4:28 PM UTC)_.

<details>
<summary>More</summary>


PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

:rotating_light: [CSS Spec Preprocessor](https://api.csswg.org/bikeshed/) - CSS Spec Preprocessor is the web service used to build Bikeshed specs.

:link: [Related URL](https://api.csswg.org/bikeshed/?url=https%3A%2F%2Fraw.githubusercontent.com%2Fwhatwg%2Fxhr%2F379930757c7286965af541f2e013342ad24776b5%2Fxhr.bs&force=1&md-status=LS-PR&md-Text-Macro=PR-NUMBER%20279)



_If you don't have enough information above to solve the error by yourself (or to understand to which web service the error is related to, if any), please [file an issue](https://github.com/tobie/pr-preview/issues/new?title=Error%20not%20surfaced%20properly&body=See%20whatwg/xhr%23279.)._
</details>

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/xhr/pull/279


-- Commit Summary --

  * use XMLHttpRequestBodyInit as defined in Fetch, in send.

-- File Changes --

    M xhr.bs (5)

-- Patch Links --

https://github.com/whatwg/xhr/pull/279.patch

https://github.com/whatwg/xhr/pull/279.diff


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