accept string filename in `savejson`

[goretkin]

Because of this check:
https://github.com/fangq/jsonlab/blob/089fc82ffab7e03a2e537b48443cdabe22464a8e/savejson.m#L138

>> savejson("", 1, 'test.json')
>> savejson("", 1, "test.json")
Error using varargin2struct
input must be in the form of ...,'name',value,... pairs or structs

Error in savejson (line 141)
   opt=varargin2struct(varargin{:});

https://www.mathworks.com/help/matlab/matlab_prog/update-your-code-to-accept-strings.html