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

[pjfanning]

Motivation

RouteTest#header(name: String) in the Scala testkit lowercased the supplied header name using the JVM default locale:

def header(name: String): Option[HttpHeader] = rawResponse.headers.find(_.is(name.toLowerCase))

HttpHeader#lowercaseName is built with toRootLowerCase, so under a Turkish default locale the two disagree: "If-Match".toLowerCase is ıf-match (dotless i) while lowercaseName is if-match. Any header name containing a capital I then silently resolves to None.

The rest of the code base already guards against this (Helpers.toRootLowerCase, CharUtils.toLowerCase for the ASCII-only paths, and HttpCharsets has a regression test in TurkishISpec). The javadsl equivalent, javadsl/testkit/TestRouteResult, already uses toRootLowerCase — only the scaladsl side was missed.

Modification

Use pekko.util.Helpers.toRootLowerCase in RouteTest#header(name).

Result

Header lookup by name in the Scala testkit is locale independent and consistent with both HttpHeader#lowercaseName and the javadsl testkit.

Tests

• Added a regression test to ScalatestRouteTestSpec that sets the default locale to tr-TR (restoring it in a finally) and looks up an If-Match header by name. It fails with None was not equal to Some(If-Match: "xyzzy") before the fix and passes after it.
• sbt "http-testkit / Test / testOnly org.apache.pekko.http.scaladsl.testkit.ScalatestRouteTestSpec" - 12 tests passed (JDK 21).
• scalafmt --mode diff-ref=origin/main - clean.
• sbt +mimaReportBinaryIssues - not run; this is a method body change with no public API or binary shape change.

References

None - found while auditing the code base for locale sensitive lowercasing