net/http: panic on misformed If-None-Match Header with http.ServeContent [1.13 backport]

[FiloSottile]

@networkimprov requested issue #39817 to be considered for backport to the next 1.13 minor release.

CL at https://golang.org/cl/240343

[dmitshur]

Backport approved for Go 1.14 (#39920) and 1.13 (this issue) as this is a serious issue with no workaround.

[gopherbot]

Change https://golang.org/cl/242077 mentions this issue: [release-branch.go1.13] net/http: fix panic with If-None-Match value in http.ServeContent

[gopherbot]

Closed by merging e19af0f to release-branch.go1.13.