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kotlinlang

I want to upload a byte stream with ktor-client. The body is set to a `ByteReadChannel`, but then the `Content-Length`-header is not set. Setting it manually throws:
```Header(s) [Content-Length] are controlled by the engine and cannot be set explicitly
io.ktor.http.UnsafeHeaderException: Header(s) [Content-Length] are controlled by the engine and cannot be set explicitly```

You can set `Content-Length` header by overriding `contentLength` property of an object of `OutgoingContent.ReadChannelContent`  class. Here is an example:
```val client = HttpClient(Apache)

val channel = ByteChannel()
channel.writeStringUtf8("hello")
channel.close()

val r = <http://client.post|client.post>&lt;String&gt;("<https://httpbin.org/post>") {
    body = object : OutgoingContent.ReadChannelContent() {
        override val contentLength: Long
            get() = 5

        override fun readFrom(): ByteReadChannel {
            return channel
        }
    }
}

println(r)```

Thank you very much. Would be cool, when this were in the Docs :)