Feature/empty text when no stripping and no collapsing enabled

Dealt with situation when with the following parameters:
```
[XMLDictionaryParser sharedInstance].collapseTextNodes = NO;
[XMLDictionaryParser sharedInstance].stripEmptyNodes = NO;
[XMLDictionaryParser sharedInstance].attributesMode = XMLDictionaryAttributesModePrefixed;
[XMLDictionaryParser sharedInstance].nodeNameMode = XMLDictionaryNodeNameModeNever;
```
The key ```__text``` was missing in the node. Now the value is empty string. 
**XML**
```
	    <emptyNode/>
	    <notEmptyNode>nodeValue</notEmptyNode>
```
**what it was before this patch**
```
emptyNode =         {
};
notEmptyNode =         {
  "__text" = nodeValue;
};
```
**After this patch**
```
emptyNode = {
"__text" = "";
};
notEmptyNode = {
"__text" = nodeValue;
};
```