fix: use root locale when looking up a response header by name in RouteTest

http-testkit/src/main/scala/org/apache/pekko/http/scaladsl/testkit/RouteTest.scala

@@ -36,6 +36,7 @@ import pekko.stream.{ Materializer, SystemMaterializer }
 import pekko.stream.scaladsl.Source
 import pekko.testkit.TestKit
 import pekko.util.ConstantFun
+import pekko.util.Helpers.toRootLowerCase
 
 import com.typesafe.config.{ Config, ConfigFactory }
 
@@ -96,7 +97,7 @@ trait RouteTest extends RequestBuilding with WSTestRequestBuilding with RouteTes
   def charset: HttpCharset = charsetOption.getOrElse(sys.error("Binary entity does not have charset"))
   def headers: immutable.Seq[HttpHeader] = rawResponse.headers
   def header[T >: Null <: HttpHeader: ClassTag]: Option[T] = rawResponse.header[T](implicitly[ClassTag[T]])
-  def header(name: String): Option[HttpHeader] = rawResponse.headers.find(_.is(name.toLowerCase))
+  def header(name: String): Option[HttpHeader] = rawResponse.headers.find(_.is(toRootLowerCase(name)))
   def status: StatusCode = rawResponse.status
 
   def closingExtension: String = chunks.lastOption match {

http-testkit/src/test/scala/org/apache/pekko/http/scaladsl/testkit/ScalatestRouteTestSpec.scala

@@ -13,6 +13,8 @@
 
 package org.apache.pekko.http.scaladsl.testkit
 
+import java.util.Locale
+
 import scala.concurrent.Await
 import scala.concurrent.Future
 import scala.concurrent.duration._
@@ -57,6 +59,25 @@ class ScalatestRouteTestSpec extends AnyFreeSpec with Matchers with ScalatestRou
       }
     }
 
+    "a header lookup by name that is unaffected by the turkish-i problem" in {
+      val previousLocale = Locale.getDefault
+      try {
+        Locale.setDefault(new Locale("tr", "TR"))
+        // in the turkish locale 'I'.toLowerCase is a dotless i, so a default-locale
+        // lowercasing of 'If-Match' would not match the header's lowercaseName
+        val ifMatchHeader = RawHeader("If-Match", "\"xyzzy\"")
+        Get() ~> {
+          respondWithHeader(ifMatchHeader) {
+            complete("abc")
+          }
+        } ~> check {
+          header("If-Match") shouldEqual Some(ifMatchHeader)
+        }
+      } finally {
+        Locale.setDefault(previousLocale)
+      }
+    }
+
     "a test using ~!> and some checks" in {
       // raw here, should have been parsed into modelled header when going through an actual server when using `~!>`
       val extraHeader = RawHeader("X-Forwarded-Proto", "abc")