httpclient.c:_RequestCallbackWrite does not handle partial sends.

[dantraMSFT]

When connecting to office 365 with my domain credentials, the size of the client data exceeds the send buffer size and a partial send occurs. _RequestCallbackWrite calls _WriteClientHeader followed by _WriteClientData. When _RequestCallbackWrite is called a second time, it again calls _WriteClientHeader which returns an error since there is no header data to send.

A possible fix is to update _RequestCallbackWrite to detect the continuation of the client data and not call _WriteClientHeader. Added a (handler->SendHeader) condition appears to solve the problem but may not be the right solution.

This is blocking New-PSSession in PowerShell Core on non-windows systems. See PowerShell/PowerShell#5130

[dantraMSFT]

NOTE: A possible fix is to make the call to _WriteClientHandler conditioned on handler->sendHeader:
if (handler->sendHeader)
{
// NOTE: for client data that requires multiple sends,
// handler->sendHeader will be cleared after the header is sent.
// Calling _WriteHeader a second time will result in an error.
switch (_WriteClientHeader(handler))
{
case PRT_CONTINUE: break;
case PRT_RETURN_TRUE: return MI_TRUE;
case PRT_RETURN_FALSE: return MI_FALSE;
}
}