fix bugs in string support

Author: petercorke

tb_optparse.m

@@ -116,15 +116,15 @@
     assert(iscell(argv), 'SMTB:tboptparse:badargs', 'input must be a cell array');
 
     if ~verLessThan('matlab', '9.1')
-        % strings only appeared in 2016b
+        % is only appeared in 2016b
 
         % handle new style string inputs
         % quick and dirty solution is to convert any string to a char array and
         % then carry on as before
 
         % convert any passed strings to character arrays
         for i=1:length(argv)
-            if isstring(argv{i})
+            if isstring(argv{i}) && length(argv{i}) == 1
                 argv{i} = char(argv{i});
             end
         end