Fixed escaping body of inboundxml Elements.

Here is the bug following the change of escaping the body_string.
The body_string variable contains the XML of all the childs which leads to this problem:
```
print inboundxml.Response(inboundxml.Say('ok'))
<Response>&lt;Say&gt;ok&lt;/Say&gt;</Response>
```
The body of each element should be escaped separately.
Please see my change.