[0.18] Fix ClassCastException in document encoding

[kubukoz]

Closes #1158.

As far as I understand, case x: ArraySeq[Document] isn't actually doing a class check because of type erasure.

The unsafe array inside turns out to actually be an Array[Object], so casting it to Array[Document] throws because arrays aren't erased (unlike ArraySeq which is a normal generic class).

I'm sure this solution is a bit inefficient, another one I had in mind would confirm the identity of the wrapped array:

a.value match {
  case x: ArraySeq[_]
      if x.unsafeArray.getClass == classOf[Array[Document]] =>
    val xs = x.unsafeArray.asInstanceOf[Array[Document]]
    var i = 0
    while (i < xs.length) {
      encodeValue(xs(i), out)
      i += 1
    }
  case xs =>
    xs.foreach(encodeValue(_, out))
}

but I'm not sure it's better as I didn't run any benchmarks. @plokhotnyuk any thoughts on this PR?

Note: the fix for the issue should be backported to 0.17.

EDIT: updated the implementation to be closer to the original, just checking for empty arrays instead. Perhaps it's worth a property-based test of roundtripping arbitrary documents just in case.