Using ServerAddress for Host header

[lehecka]

The HttpClient can be initialized with a ServerName(string) or ServerAddress (IPAddress). In the first case the underlaying Client will use DNS lookup to translate the name to the IP address. This lookup code is consuming extra bytes in memory. When making the HTTP request the ServerName is used for Host header.
If the ServerAddress is specified in the ctor of the HttpClient then the HTTP requests are missing the Host header.

In this diff the user has ability to call useServerAddressForHostHeader to use the ServerAddress for the Host header.

As a workaround without this fix one has to use sendHeader / beginBody / endRequest call which makes the API use very cumbersome and error prone .

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