(MODULES-905) Narrow the confinement in bool2str

Previously, bool2str() accepted a broad array of boolean values and
bare strings, without any attempt to validate that the strings in any
way resembled "true" or "false" (or any of the other values bool2num()
accepts). This commit narrows the input confinement to TrueClass and
FalseClass, which means that bool2str() will only interpolate
strict boolean values now.