Added missing ability of specifying value and the same time an attributes for simple tag 

Given we want to generate xml contains this:

    <tag attr='value'>Text</tag>

In Savon:

we can do only
  
    operation.body = {
      tag: { :_attr => 'value'}
    }

or

    operation.body = {
      tag: 'Text'
    }   

For some reason, Savon doesn't handle that case when we want to specify value and attributes for tag.


Trying to handle that case in PR like so:

    operation.body = {
      tag: {tag: 'Text', :_attr => 'value'}
    }   

Will produced an expected xml.

Please, review a PR. 

Thanks.