IIS7 with ARR as reverse proxy and non-ACSII querystring

I just installed Windows Server 2008 R2 with IIS, ARR 2.5 and URL Rewrite 2 as reverse proxy. Unfortunately proxy seems to modify non-ACSII querystrings.

I setup testing web page wp.imperium.cz/index.asp. This page just displays received querystring. And result fromthis request: http://wp.imperium.cz/?asd=Tomáš is

Query string is: asd=Tomáš

My rewrite rule in applicationHost:

<rule name="ARR_webfarm_loadbalance" enabled="true" patternSyntax="Wildcard" stopProcessing="true">
                    <match url="*" ignoreCase="true" />
                    <action type="Rewrite" url="http://webfarm/{R:0}" />
                    <conditions logicalGrouping="MatchAll">
                        <add input="{LOCAL_ADDR}" pattern="x.x.x.x" />
                    </conditions>
</rule>

 Is there any way how I can configure my proxy for non-ASCII querystring support?