premature "Nextpart: EOF" on POST /api/v0/add request

### Checklist

- [X] This is a bug report, not a question. Ask questions on [discuss.ipfs.io](https://discuss.ipfs.io).
- [X] I have searched on the [issue tracker](https://github.com/ipfs/go-ipfs/issues?q=is%3Aissue) for my bug.
- [X] I am running the latest [go-ipfs version](https://dist.ipfs.io/#go-ipfs) or have an issue updating.

### Installation method

built from source

### Version

```Text
Installation method: nix-env -i ipfs

go-ipfs version: 0.10.0
Repo version: 11
System version: arm64/linux
Golang version: go1.16.9
```

### Config

```json
{
"API": {
"HTTPHeaders": {
"Access-Control-Allow-Methods": [
"PUT",
"POST"
],
"Access-Control-Allow-Origin": [
"http://:5001"
]
}
},
"Addresses": {
"API": "/ip4/0.0.0.0/tcp/5001",
"Announce": [],
"Gateway": "/ip4/127.0.0.1/tcp/8080",
"NoAnnounce": [],
"Swarm": [
"/ip4/0.0.0.0/tcp/4001",
"/ip6/::/tcp/4001",
"/ip4/0.0.0.0/udp/4001/quic",
"/ip6/::/udp/4001/quic"
]
},
"AutoNAT": {},
"Bootstrap": [],
"DNS": {
"Resolvers": {}
},
"Datastore": {
"BloomFilterSize": 0,
"GCPeriod": "1h",
"HashOnRead": false,
"Spec": {
"mounts": [
{
"child": {
"path": "blocks",
"shardFunc": "/repo/flatfs/shard/v1/next-to-last/2",
"sync": true,
"type": "flatfs"
},
"mountpoint": "/blocks",
"prefix": "flatfs.datastore",
"type": "measure"
},
{
"child": {
"compression": "none",
"path": "datastore",
"type": "levelds"
},
"mountpoint": "/",
"prefix": "leveldb.datastore",
"type": "measure"
}
],
"type": "mount"
},
"StorageGCWatermark": 90,
"StorageMax": "10GB"
},
"Discovery": {
"MDNS": {
"Enabled": true,
"Interval": 10
}
},
"Experimental": {
"AcceleratedDHTClient": false,
"FilestoreEnabled": false,
"GraphsyncEnabled": false,
"Libp2pStreamMounting": false,
"P2pHttpProxy": false,
"ShardingEnabled": false,
"StrategicProviding": false,
"UrlstoreEnabled": false
},
"Gateway": {
"APICommands": [],
"HTTPHeaders": {
"Access-Control-Allow-Headers": [
"X-Requested-With",
"Range",
"User-Agent"
],
"Access-Control-Allow-Methods": [
"GET"
],
"Access-Control-Allow-Origin": [
"*"
]
},
"NoDNSLink": false,
"NoFetch": false,
"PathPrefixes": [],
"PublicGateways": null,
"RootRedirect": "",
"Writable": false
},
"Identity": {
"PeerID": ""
},
"Internal": {},
"Ipns": {
"RecordLifetime": "",
"RepublishPeriod": "",
"ResolveCacheSize": 128
},
"Migration": {
"DownloadSources": [],
"Keep": ""
},
"Mounts": {
"FuseAllowOther": false,
"IPFS": "/ipfs",
"IPNS": "/ipns"
},
"Peering": {
"Peers": []
},
"Pinning": {
"RemoteServices": {}
},
"Plugins": {
"Plugins": null
},
"Provider": {
"Strategy": ""
},
"Pubsub": {
"DisableSigning": false,
"Router": ""
},
"Reprovider": {
"Interval": "12h",
"Strategy": "all"
},
"Routing": {
"Type": "dht"
},
"Swarm": {
"AddrFilters": null,
"ConnMgr": {
"GracePeriod": "30s",
"HighWater": 500,
"LowWater": 300,
"Type": "basic"
},
"DisableBandwidthMetrics": false,
"DisableNatPortMap": false,
"EnableAutoRelay": false,
"EnableRelayHop": false,
"Transports": {
"Multiplexers": {},
"Network": {},
"Security": {}
}
}
}
```

### Description

`POST /api/v0/add` will prematurely respond with "Nextpart: EOF" before even the first part has been sent if the HTTP request is split into multiple packets. `/ipfs/QmXNuNB13zYS6WDAjkq8YG6mPtdni9xUNrcSzX6xufda8Z/broken.pcap` shows a packet dump. note that the HTTP header set comes in two packets, after the second the API responds with the error before the HTTP request even had a chance to send the first part of the multipart request.
by comparison `/ipfs/QmcNL9Dv9XQ6GBDmHmaHByAhNUe5bNCNPkgpMTaDgqpJrP/works.pcap` is the exact same situation except that all HTTP headers come in one packet.

the latter is IPFS command line standard behavior. the former (broken) can e.g. per provoked by inserting an HTTP proxy like tinyproxy in the middle.