Fix #102: Wrong options.columns initialization.

src/bootstrap-table.js

@@ -269,7 +269,7 @@
 
             columns.push(column);
         });
-        this.options.columns = $.extend({}, columns, this.options.columns);
+        this.options.columns = $.extend([], columns, this.options.columns);
         $.each(this.options.columns, function (i, column) {
             that.options.columns[i] = $.extend({}, BootstrapTable.COLUMN_DEFAULTS,
                 {field: i}, column); // when field is undefined, use index instead