HTTP API reports incorrect size after adding via /api/v0/add

#### Version information:
```
$ ipfs version --all
go-ipfs version: 0.4.15-
Repo version: 6
System version: amd64/darwin
Golang version: go1.10
```

#### Type:
Bug

#### Description:
The HTTP API reports an incorrect size for a file added to IPFS via the `/api/v0/add` endpoint.

To reproduce:
0) Clean slate: `rm -rf ~/.ipfs && ipfs init && ipfs daemon &`
1) Create a file named `testfile` containing just the contents:`Plz add me!`. This is exactly 12 bytes.
2) `POST` to `/api/v0/add` to add the file.
3) Observe that the response says the `Size` is 20.
4) Read it back via `ipfs cat Qma4hjFTnCasJ8PVp3mZbZK5g2vGDT4LByLJ7m8ciyRFZP | wc -c` and observe it was correctly stored with its 12 byte length.

Sample session follows:
```
$ rm -rf ~/.ipfs && ipfs init && ipfs daemon &
initializing IPFS node at /Users/io/.ipfs
generating 2048-bit RSA keypair...done
peer identity: Qmc7dLt8afHiDtoE3VabiGUUiq637X8i8CPCkx936C6URM
to get started, enter:

ipfs cat /ipfs/QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv/readme

[1] 12968
Initializing daemon...
Swarm listening on /ip4/10.105.46.40/tcp/4001
Swarm listening on /ip4/10.140.0.2/tcp/4001
Swarm listening on /ip4/127.0.0.1/tcp/4001
Swarm listening on /ip6/::1/tcp/4001
Swarm listening on /p2p-circuit/ipfs/Qmc7dLt8afHiDtoE3VabiGUUiq637X8i8CPCkx936C6URM
Swarm announcing /ip4/10.105.46.40/tcp/4001
Swarm announcing /ip4/10.140.0.2/tcp/4001
Swarm announcing /ip4/127.0.0.1/tcp/4001
Swarm announcing /ip6/::1/tcp/4001
API server listening on /ip4/127.0.0.1/tcp/5001
Gateway (readonly) server listening on /ip4/127.0.0.1/tcp/8080
Daemon is ready
$ cat testfile
Plz add me!
$ cat testfile | wc -c
12
$ cat testfile | wc -l
1
$ cat testfile | xxd
00000000: 506c 7a20 6164 6420 6d65 210a Plz add me!.
$ ipfs add testfile
added Qma4hjFTnCasJ8PVp3mZbZK5g2vGDT4LByLJ7m8ciyRFZP testfile
$ curl -F arg=@testfile "http://localhost:5001/api/v0/add"
{"Name":"testfile","Hash":"Qma4hjFTnCasJ8PVp3mZbZK5g2vGDT4LByLJ7m8ciyRFZP","Size":"20"}
$ ipfs cat Qma4hjFTnCasJ8PVp3mZbZK5g2vGDT4LByLJ7m8ciyRFZP | wc -c
12
$ ipfs cat Qma4hjFTnCasJ8PVp3mZbZK5g2vGDT4LByLJ7m8ciyRFZP | xxd
00000000: 506c 7a20 6164 6420 6d65 210a Plz add me!.
```
Note that building a 0.4.16 master correctly shows `12 B` in the `ipfs add` command, but still reports `"Size": "20"` over the HTTP API.