Miscellaneous GML and WFS fixes

This PR fixes a few things in `ol.format.GML` and `ol.format.WFS`. I'm not very familiar with either format, so please ignore or cherry-pick as appropriate.

The four commits are, in order:

* A tightening of the return types in `ol.format.WFS#writeGetFeature` (it always returns a `Node`)
* A fix to the way that ol3 undoes the OGC's stupid mangling of the coordinate order (`reverse()` only works with 2D coordinates)
* Parsing coordinates in a single pass, rather than multiple passes
* A few tests, that I expected would pass, but did not (maybe due to my lack of knowledge of GML)

CC @bartvde