Fix a damn bug cause by parsing XML.

The XML return by Yahoo API is in the form:

<categories count="8">
    <category>
        <no>259</no>
        <level_no>2</level_no>
        <name><![CDATA[ohoh-mini]]></name>
        <type><![CDATA[1]]></type>
    </category>
    <category>
        <no>260</no>
        <level_no>2</level_no>
        <name><![CDATA[ohoh-max]]></name>
        <type><![CDATA[2]]></type>
    </category>
</categories>

After parsing the XML(via MultiXML), we get the Hash in the form:

{"categories"=>
    {"count"=>"8", "category"=>
        [{"no"=>"259", "level_no"=>"2", "name"=>"ohoh-mini", "type"=>"1"}, {"no"=>"260", "level_no"=>"2", "name"=>"ohoh-max", "type"=>"2"}]
    }
}

The bug happens when the returned XML is:

<categories count="8">
    <category>
        <no>259</no>
        <level_no>2</level_no>
        <name><![CDATA[ohoh-mini]]></name>
        <type><![CDATA[1]]></type>
    </category>
</categories>

In this case, the Hash will be:

{"categories"=>{"category"=>{"no"=>"259", "level_no"=>"2", "name"=>"ohoh-mini", "type"=>"1"}, "count"=>"8"}}

i.e. the value of key "category" will be a Hash instead of an Array, this will cause an error in Forklift::Utils