Preserve request precondition headers in NoCache

[puneetdixit200]

Summary

• stop NoCache from deleting conditional request headers such as If-Match and If-None-Match
• add a regression test that confirms precondition headers reach downstream handlers while no-cache response headers are still set

Fixes #801.

Tests

• GOMODCACHE=/tmp/chi-801-gomodcache GOCACHE=/tmp/chi-801-gocache go test ./middleware -run TestNoCachePreservesRequestPreconditionHeaders -count=1
• GOMODCACHE=/tmp/chi-801-gomodcache GOCACHE=/tmp/chi-801-gocache go test ./middleware -run 'NoCache|TestNoCachePreservesRequestPreconditionHeaders' -count=1
• GOMODCACHE=/tmp/chi-801-gomodcache GOCACHE=/tmp/chi-801-gocache go test ./middleware -count=1
• GOMODCACHE=/tmp/chi-801-gomodcache GOCACHE=/tmp/chi-801-gocache go test ./... -count=1
• git diff --check