fix: real unmodifiable list

[MukjepScarlet]

fixes #8721

[MukjepScarlet]

@JakeWharton Please review

[swankjesse]

Why make our code slower?

Here’s the Java equivalent of what we had before (IntelliJ ‘Show Kotlin Bytecode’, followed by ‘Decompile’):

   public static final long headersContentLength(@NotNull Response $this$headersContentLength) {
      Intrinsics.checkNotNullParameter($this$headersContentLength, "<this>");
      String var10000 = $this$headersContentLength.headers().get("Content-Length");
      return var10000 != null ? _UtilCommonKt.toLongOrDefault(var10000, -1L) : -1L;
   }

and here’s after:

   public static final long headersContentLength(@NotNull Response $this$headersContentLength) {
      Intrinsics.checkNotNullParameter($this$headersContentLength, "<this>");
      String var10000 = $this$headersContentLength.headers().get("Content-Length");
      long var2;
      if (var10000 != null) {
         Long var1 = StringsKt.toLongOrNull(var10000);
         if (var1 != null) {
            var2 = var1;
            return var2;
         }
      }

      var2 = -1L;
      return var2;
   }

The old code didn’t allocate, and the new code does.

[swankjesse]

#8801