http/httpguts: reject leading and trailing spaces in field values

[DemiMarie]

RFC9113 is extremely clear about this:

A field value MUST NOT start or end with an ASCII whitespace character
(ASCII SP or HTAB, 0x20 or 0x09).

RFC9114 defers to RFC9110, which in turn provides a grammer that does not permit leading and/or trailing whitespace either.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 27695fa) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/net/+/665175.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
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• To change your code in response to feedback:
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[gopherbot]

Message from Damien Neil:

Patch Set 2: Hold+1

(1 comment)

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[gopherbot]

Message from Demi Obenour:

Patch Set 2:

(1 comment)

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