I have solved this issue by removing all children tags from SignedParts tag
So my policy definition looks like this.
<wsp:ExactlyOne>
<wsp:All>
<sp:SignedParts xmlns:sp="http://schemas.xmlsoap.org/ws/2005/07/securitypolicy</sp:SignedParts>
</wsp:All>
</wsp:ExactlyOne>
I've found out that as long as you don't define any tags for signing and you include just empty tag SignedParts it will automatically sign all headers and body.