WMS 1.3.0 Boolean representation

[pomadchin]

In terms of #311 it was revealed that OWSLib represents Booleans as {0, 1} of type Integer. At this point the workaround was to skip Booleans in all WMS endpoints, but in the future we may want to use these extra parameters.

This task is to track the status of this issue. That was an appropriate OWSLib restriction for the WMS 1.1.x, but it could be legitimate to represent Booleans as strings ("true" / "false") in terms of the WMS 1.3.x.

[pomadchin]

It looks like the majority of libraries work with WMS Booleans as with 0 and 1 (QGIS is an example of such library). I think it makes sense for us to force {0, 1} usage for all WMS endpoints.