fun <T : Any> append(key: String, value: T, headers: Headers = Headers.Empty): Unit
fun append(key: String, value: String, headers: Headers = Headers.Empty): Unit
fun append(key: String, value: Number, headers: Headers = Headers.Empty): Unit
fun append(key: String, value: ByteArray, headers: Headers = Headers.Empty): Unit
fun append(key: String, value: InputProvider, headers: Headers = Headers.Empty): Unit
fun append(key: String, value: ByteReadPacket, headers: Headers = Headers.Empty): Unit
fun append(key: String, value: Input, headers: Headers = Headers.Empty): Unit
Append a pair key:value with optional headers.
fun <T : Any> append(part: FormPart<T>): Unit
Append a form part.