When field is undefined, use index instead

docs/examples.html

@@ -179,9 +179,9 @@
                     <table id="table-transform" data-toolbar="#transform-buttons">
                         <thead>
                         <tr>
-                            <th data-field="id">Item ID</th>
-                            <th data-field="name">Item Name</th>
-                            <th data-field="price">Item Price</th>
+                            <th>Item ID</th>
+                            <th>Item Name</th>
+                            <th>Item Price</th>
                         </tr>
                         </thead>
                         <tbody>

src/bootstrap-table.js

@@ -248,7 +248,8 @@
         });
         this.options.columns = $.extend({}, columns, this.options.columns);
         $.each(this.options.columns, function (i, column) {
-            that.options.columns[i] = $.extend({}, BootstrapTable.COLUMN_DEFAULTS, column);
+            that.options.columns[i] = $.extend({}, BootstrapTable.COLUMN_DEFAULTS,
+                {field: i}, column); // when field is undefined, use index instead
         });
 
         // if options.data is setting, do not process tbody data