Better validation of attributes and char content when writting XML

[jhorbulyk]

Consider the following code:

const xml2js = require('xml2js');
const builder = new xml2js.Builder({
    headless: true,
});
sampleObject1 = {
    "root": {
        "$": {
            "foo": {"bar": "baz"}
        }
    }
}
console.log(`Sample A: \n${builder.buildObject(sampleObject1)}\n`);

sampleObject2 = {
    "root": {"_": {"foo":"bar"}}
}
console.log(`Sample B: \n${builder.buildObject(sampleObject2)}\n`);

Observe that the output is

Sample A: 
<root foo="[object Object]"/>

Sample B: 
<root>[object Object]</root>

In both cases, the input isn't logical. Any value supplied to attrKey at any level should be a simple object with out nested arrays or sub-objects. Likewise all charkey values should be a string. That isn't the case with these two examples which is why they produce strange outputs. However, ideally, there should be some sort of validation to check for these cases and produce errors in these cases.