Don't convert NSString to String using format:

The make(a: NSObject) method attempts to convert its NSString value to
a String value by passing it to String(format:) and never provides any
arguments. Therefore it fails to decode any string that contains a
% character. This changes it to call to String(_).
Merge pull request #139 from mgp/master

Don't convert NSString to String using format:
Merge remote-tracking branch 'upstream/master'