fix bug in sefun/border->format_border where it added a '0' instead of right wall symbol and update tests for new sefun/color->color_gradient method

Author: michaelprograms

lib/secure/sefun/border.c

@@ -260,7 +260,7 @@ string *format_border (mapping rawData, mapping b, int width, string ansi) {
                 line[1+fHeader..2] +
                 SEFUN->apply_gradient(line[3..l+1], colors[3..l+1]) +
                 line[l+2..<2+fHeader] +
-                SEFUN->apply_gradient(line[<1+fHeader..<1], colors[<1+fHeader..<1]);
+                SEFUN->apply_gradient(line[<1+fHeader..<1], colors[<1+fHeader..<1]) + ""; // empty string at the end prevents a strange bug where it inserts '0' instead of correct right wall symbol
         } else if (ansi) {
             line = "\e[36m" + line[0..fHeader] + "\e[0;37;40m" +
             line[1+fHeader..2] +