Use set for sensitiveHeaders

[ribice]

Replaced bool with empty struct to better represent a set, which is what SensitiveHeaders is.

func BenchmarkNewRequest(b *testing.B) {
	const payload = `{"test_data": true}`
	r := httptest.NewRequest("POST", "/test/?q=sentry", strings.NewReader(payload))
	r.Header.Add("Authorization", "Bearer 1234567890")
	r.Header.Add("Cookie", "foo=bar")
	r.Header.Add("X-Forwarded-For", "127.0.0.1")
	r.Header.Add("X-Real-Ip", "127.0.0.1")
	r.Header.Add("Some-Header", "some-header value")

	b.ReportAllocs()
	for i := 0; i < b.N; i++ {
		NewRequest(r)
	}
}

Before:

BenchmarkNewRequest-10    	 2724643	       430.9 ns/op	     504 B/op	       7 allocs/op

After:

BenchmarkNewRequest-10    	 2993946	       394.1 ns/op	     504 B/op	       7 allocs/op

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.14%. Comparing base (c10f51e) to head (fae152e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #802      +/-   ##
==========================================
- Coverage   81.17%   81.14%   -0.04%     
==========================================
  Files          49       49              
  Lines        4127     4120       -7     
==========================================
- Hits         3350     3343       -7     
  Misses        635      635              
  Partials      142      142              

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[cleptric]

lgtm