#663 nathelper: fix_nated_sdp appends two extra oldmediaip attributes

[Ovidiu Sas]

Adding oldmediaip attribute should be controlled by a flag (0x16).
If the flag is present, the attribute should be appended only once (controlled by 'preserve' param of 'alter_mediaip' function).

Regards,
Ovidiu Sas

[Bogdan-Andrei Iancu]

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