[whatwg/fetch] Pragma in responses (#1018)

The [CORS safelisted responses headers list](https://fetch.spec.whatwg.org/#cors-safelisted-response-header-name) includes `Pragma`.

[Section 5.4 of RFC 7234](https://httpwg.org/specs/rfc7234.html#header.pragma) specifically states that `Pragma` only has semantics in requests.

On the other side of this, we recently discovered a few cases where `Pragma` was used instead of `Cache-Control`.  That is inadvisable, but it does seem to be in use still; how much is redundant with Cache-Control is hard to know.  Chrome treats `Pragma: no-cache` like `Cache-Control: no-cache` in both request and response.

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