Fix for #846 (extremely quick and dirty).

** DO NOT MERGE** until @ggalibert says that this change is functionally correct.

Ensure that for wms download, we use the time coverage columns from the WMS layer (not the WFS layer, as was previously happening).

Note that there are some excellent examples of cut-and-paste coding in this PR which I intent to factor out, but wanted to get this quick and dirty fix in ASAP because of the impending release.