Make caption arg in submit-tag helper optional even when options args are supplied

When I use `FormHelpers#submit_tag` without caption arg,
```ruby
submit_tag :class => "btn"
```
I expect
```html
<input type="submit" class="btn" value="Submit" />
```
But I get
```html
<input type="submit" value-class="btn" />
```
I know that a caption arg is mandatory when options args are specified, but I think it's useful if we can omit a caption arg.


(Furthermore, IMHO, on above example condition, outputting as follows would be reasonable  (`value` attr is omitted).  In this pull-req I did'nt touch so, but how do you think about this idea?)
```html
<input type="submit" class="btn" />
```