too strict attributes validation

[rlidwka]

Right now this module expect attributes to be string primitives.

I'd expect numbers to be automatically converted to strings if necessary. There is also a common practice how to handle boolean and non-existent attributes (see below).

// this fails, expected: <element test="123">
xml.startElement('element').writeAttribute('test', 123)

// this fails, expected: <element test="123">
xml.startElement('element').writeAttribute('test', new String('123'))

// this fails, expected: <element test="test">
xml.startElement('element').writeAttribute('test', true)

// this fails, expected: <element>
xml.startElement('element').writeAttribute('test', false)

// this fails, expected: <element>
xml.startElement('element').writeAttribute('test', null)

// this fails, expected: <element>
xml.startElement('element').writeAttribute('test', undefined)

[touv]

There are a small function to check the type of all parameters. cf. https://github.com/lindory-project/node-xml-writer/blob/master/lib/xml-writer.js#L2

This function can be improve. Any pull request are welcome.