FIX DEV-41949: LLMediaEntry::setWhitelist() and LLMediaEntry::asLLSD() have a contract conflict

Review #32

This code will go into both server 1.32 branch and the viewer branch.

As proposed in DEV-41949, I've changed setWhitelist(const LLSD &whitelist) to clear the whitelist if the WHITELIST_KEY is not present in the passed-in 'whitelist'. I've also made sure that asLLSD() ensures that the WHITELIST_KEY is erased in the given LLSD.