net/http: fix Transport panic with nil Request.Header

For Go 1.13 we introduced Header.Clone and it returns
nil if a nil Header is cloned. Unfortunately, though,
this exported Header.Clone nil behavior differed from
the old Go 1.12 and earlier internal header clone
behavior which always returned non-nil Headers.
This CL fixes the places where that distinction mattered.

Fixes #34878

Change-Id: Id19dea2272948c8dd10883b18ea7f7b8b33ea8eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/200977
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed
tree: af45c6920f24ee26fc0b5931c020c2de0d706d2b
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
  18. SECURITY.md
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