Correct handling of 0 and Infinity ScaleHint

minScale and maxScale values of 0 and Infinity do not make sense in
OpenLayers, but they are used in WMS GetCapabilities 1.1.1 to indicate
that there is no minScale or maxScale restriction. Assuming that the
capability values for minScale and maxScale are passed as config options
to a WMS layer, it is better to not set minScale and maxScale when 0 or
Infinity are found in the ScaleHint.
Documenting ScaleHint parsing, as suggested by @marcjansen