Improve string handling in xsl back calls

Sometimes XSL sends a node collection instead of a String; 
this code change tries to squash the complex object into a String.

Without this code change in java, you have to perform some workarounds in the XSL code itself, 
so, instead of writing
```
 <xsl:variable name="name" select="@name"/>
 ```
you have to transform the string with 
```
   <xsl:variable name="name" select="concat('',@name)"/>
```
Other use cases implies the use of more complex workaround in XSL code.