PUT existing file with Writeable Gateway not returning expected result.

#### Version information:
go-ipfs version: 0.4.8-
Repo version: 5
System version: amd64/linux
Golang version: go1.8

#### Type:
Bug

#### Severity:
Medium

#### Description:
When using a writeable HTTP gateway:

* If I PUT to a directory tree for a path that *doesn't* exist in the tree, the gateway returns the expected result: the `ipfs-hash` for an edited directory tree that contains the uploaded file at the specified path.
* **However**, if I PUT to a directory tree for a path that *already* exists in the tree, I instead get back the `ipfs-hash` for the *file* I uploaded- as if I had used the POST interface.
* I would expect, in that case, to receive the `ipfs-hash` of the root of an edited directory tree that contains the uploaded file in place of the path's former content.