bug: index off by one

Very minor bug but forceTrailingSlash always adds a / if set to true because the index was off by one.

Author: dremonkey

src/index.js

@@ -450,7 +450,7 @@
           var _this = this;
           var start = new Date();
           config = deepMixIn(config, _this.defaults.httpConfig);
-          if (_this.defaults.forceTrailingSlash && config.url[config.url.length] !== '/') {
+          if (_this.defaults.forceTrailingSlash && config.url[config.url.length-1] !== '/') {
             config.url += '/';
           }
           config.method = config.method.toUpperCase();