🐛 Fix false positive in border-zero

This fixes a false positive when `convention` is set to the integer `0` (as opposed to the string, which does work), by casting the `convention` to a string.

Fixes sasstools#861

lib/rules/border-zero.js

@@ -29,7 +29,7 @@ module.exports = {
             var node = item.content[0];
             if (node.type === 'number' || node.type === 'ident') {
               if (node.content === '0' || node.content === 'none') {
-                if (parser.options.convention !== node.content) {
+                if (('' + parser.options.convention) !== node.content) {
                   result = helpers.addUnique(result, {
                     'ruleId': parser.rule.name,
                     'line': node.start.line,