cider-format-region does not respect indentation of region start position

e.g. if I have

``` clojure
(let [x 1]
  (let [x 1]
    (let [x 1]
      (let [x 1]
                  (println x)))))
```

and I select the innermost `let` form, then do `cider-format-region`, I get:

``` clojure
(let [x 1]
  (let [x 1]
    (let [x 1]
      (let [x 1]
  (println x)))))
```
