fix encoding issue in blogspam plugin

RPC::XML uses ascii as default encoding, we have to tell it to use utf8.

Without this, ikiwiki returns "failed to get response from blogspam server"
every time a non-ascii character is used in a content that needs checking.

IkiWiki/Plugin/blogspam.pm

@@ -53,6 +53,7 @@ sub checkconfig () {
 	eval q{
 		use RPC::XML;
 		use RPC::XML::Client;
+		$RPC::XML::ENCODING = 'utf-8';
 	};
 	error $@ if $@;
 }