TestRequest have been overrriding request_method setter since 2009,
but the actual implementation in Request (not TestRequest) has been
changed since that. Now it's also using @request_method instance
variable to keep the state.

The override in TestRequest have not been calling `super`, which caused
a bug that after accessing #requst_method the value was memoized and
then we've never been able to change it anymore:

```
req = ActionDispatch::TestRequest.create
puts "was: #{req.request_method}" # memoized here
req.request_method = "POST"
puts "became: #{req.request_method}"
```

output:

```
was: GET
became: GET
```

Since the whole purpose of overriding the setter in TestRequest is to
upcase it, I'm changing it to `super(method.to_s.upcase)`