Dft namespace fix

Fixes the case where:
```xml
<manifest xmlns="http://www.imsglobal.org/xsd/imscp_v1p1" xmlns:eps="http://eps.pearson.com/xsd/item-metadata">
 <xml xmlns="http://eps.pearson.com/xsd/item-metadata"/>
</manifest>
```
comes out as:

```xml
<manifest xmlns="http://www.imsglobal.org/xsd/imscp_v1p1" xmlns:eps="http://eps.pearson.com/xsd/item-metadata">
 <xml/>
</manifest>
```

because the non-prefixed namespace matched the uri of an parent prefixed namespace. It should only ignore the namespace if declaration if it matches exactly.