Improve consistency when explicitArray + mergeAttrs are both true

When the explicitArray and mergeAttrs options are both set, ensure
that nodes introduced by attributes are put into arrays.
Use assert.strictEqual instead of assert.equal for tests

The coercions performed by assert.equal (which uses ==) make it
difficult to see whether the output is really as intended, since,
e.g., the effect of explicitArray is not visible.
Improved consistenzy with explicitArray + mergeAttrs

Many thanks to @hoelzl for spotting and fixing this.

Merge branch 'hoelzl-master'

Conflicts:
	lib/xml2js.js