Allow strings as response without a wrapping tag.

[jesusabarca]

While constructing a SOAP service we were required to include a simple string in the response without wrapping it in another tag, like:

<MethodNameResponse>Response string</MethodNameResponse>

But by default, wash_out will wrap the string in a "Value" tag, like:

<MethodNameResponse>
  <Value>Response string</Value>
</MethodNameResponse>

I'll submit a PR with the solution I came up with.