fix issues #34 #39 #44 and #45, support double-quited strings

Author: fangq

savejson.m

@@ -171,6 +171,8 @@
     txt=struct2json(name,item,level,varargin{:});
 elseif(ischar(item))
     txt=str2json(name,item,level,varargin{:});
+elseif(isa(item,'string'))
+    txt=str2json(name,item{:},level,varargin{:});
 elseif(isobject(item)) 
     txt=matlabobject2json(name,item,level,varargin{:});
 else