Do properly represent DateTime as UTC on the client

[Tarmil]

The use of Date.getTime() in the proxy meant that DateTime values weren't always represented as UTC. This fixes this discrepancy. It might also fix remoting #636; I haven't yet tested with a server in another time zone.

[CLAassistant]

All committers have signed the CLA.

[Jand42]

Closing old PR. We might revisit DateTime representation after asymmetric RPC protocol has been retired.