Handle case where Feature booleans are 'true' or 'false' during parsing

I've come across a few kml documents that use string 'true' or 'false' for the visibility and open boolean values instead of 1 or 0.  The current way of casting to int doesn't work for these.  The changes I made check for the string values and force to integer 1 or 0. 

I'm not sure this is the best solution as it doesn't preserve the original string values when using to_string, but it works for my needs.  I thought this might be useful for others as well.