Fix a bug causing element having text node child to be considered as leaf nodes

The bug behaviour, for entity like this one:
```
<us-gaap:AccountsPayableCurrent contextRef="I2012Q4" decimals="-6" id="Fact-84774E2BABC3C1BE0251D7A597ED2610" unitRef="usd">65000000</us-gaap:AccountsPayableCurrent>
```
xCurator “attributize” method converts the attributes to child elements:
```
<us-gaap:AccountsPayableCurrent contextRef="I2012Q4" decimals="-6"
    id="Fact-84774E2BABC3C1BE0251D7A597ED2610" unitRef="usd”>65000000
    <contextRef>I2012Q4</contextRef>
    <decimals>-6</decimals>
    <id>Fact-84774E2BABC3C1BE0251D7A597ED2610</id>
    <unitRef>usd</unitRef>
</us-gaap:AccountsPayableCurrent>
```
Now the BasicSchemaExtraction step still considered this new element as a leaf node, and stopped further extracting its underlying schema.

I changed the ifLeaf implementation so that only element having no child element is leaf node. 

Note that the text value of the original element is still a text node instead of an element.