Support expanded support for converting to boolean values.

[jloveridge]

Be a bit more permissive on what values will result in a true from the boolean conversion. Additionally this fixes an issue where an exception is thrown if the value being converted happens to be null.

// examples where this could have caused an exception
const booleanConversionThrows = {
  propIsNull: JSON.stringify({value: null}),
  propIsNaN: JSON.stringify({value: NaN}),
};

[coveralls]

Coverage decreased (-5.8%) to 94.231% when pulling e0cc45d on gradecam:feature/expanded-boolean into 109f38a on bitrinjani:master.