Handle items newly deprecated in Go 1.11

Remove where possible and ignore staticcheck warnings where we have
to.

Signed-off-by: beorn7 <beorn@soundcloud.com>

Makefile

@@ -13,6 +13,13 @@
 
 include Makefile.common
 
+# http.CloseNotifier is deprecated but we don't want to remove support
+# from client_golang to not break anybody still using it.
+STATICCHECK_IGNORE = \
+  github.com/prometheus/client_golang/prometheus/promhttp/delegator*.go:SA1019 \
+  github.com/prometheus/client_golang/prometheus/promhttp/instrument_server_test.go:SA1019 \
+  github.com/prometheus/client_golang/prometheus/http.go:SA1019
+
 .PHONY: get_dep
 get_dep:
 	@echo ">> getting dependencies" 

prometheus/registry_test.go

@@ -671,7 +671,7 @@ collected metric named "complex_count" collides with previously collected histog
 		handler(writer, request)
 
 		for key, value := range scenario.out.headers {
-			if writer.HeaderMap.Get(key) != value {
+			if writer.Header().Get(key) != value {
 				t.Errorf(
 					"%d. expected %q for header %q, got %q",
 					i, value, key, writer.Header().Get(key),