fix nesting errors in Nokogiri xml_processor

I do, as you write in the instructions (readme.rdoc)
```
OpenSRS::Server.xml_processor = :nokogiri

server_local = OpenSRS::Server.new(:server   => "http://localhost:3000/opensrs",
                                   :username => "username",
                                   :password => "53cr3t",
                                   :key      => "c633be3170c7fb3fb29e2f99b84be2410" )

  def get_some_action(some_index)
    server_local.call(
      :action => "action_name",
      :object => "object_name",
      :attributes => {
        :some_index => some_index
      }
    )
  end

get_some_action("578").request_xml
```
after this I get XML request like this:
```
<?xml version="1.0"?>
<OPS_envelope>
  <header/>
  <version/>
  0.9
  <body/>
  <data_block/>
  <dt_assoc>
    <item key="protocol">XCP</item>
    <item key="action">action_name</item>
    <item key="object">object_name</item>
    <item key="attributes">
    <dt_assoc>
      <item key="some_index">578</item>
    </dt_assoc>
    </item>
  </dt_assoc>
</OPS_envelope>
```
But in my opinion it must be like this
```
<?xml version="1.0"?>
<OPS_envelope>
  <header>
    <version>0.9</version>
  </header>
  <body>
    <data_block>
      <dt_assoc>
        <item key="protocol">XCP</item>
        <item key="action">action_name</item>
        <item key="object">object_name</item>
        <item key="attributes">
          <dt_assoc>
            <item key="some_index">578</item>
          </dt_assoc>
        </item>
      </dt_assoc>
    </data_block>
  </body>
</OPS_envelope>
```
if I change xml_processor to libxml
```
OpenSRS::Server.xml_processor = :libxml
```
I get correct xml request
```
<?xml version="1.0" encoding="UTF-8"?>
<OPS_envelope>
  <header>
    <version>0.9</version>
  </header>
  <body>
    <data_block>
      <dt_assoc>
        <item key="protocol">XCP</item>
        <item key="action">action_name</item>
        <item key="object">object_name</item>
        <item key="attributes">
          <dt_assoc>
            <item key="some_index">578</item>
          </dt_assoc>
        </item>
      </dt_assoc>
    </data_block>
  </body>
</OPS_envelope>
```