Have allowDuplicateKeys default to true

[twyatt]

Change the default to match Android's behavior (the intention is that Kable out-of-the-box/with defaults, should give as consistent behavior as possible across platforms).

Without this change (with allowDuplicateKeys using Core Bluetooth's default, which is false) only 1 or 2 advertisement events are received during a scan, and doesn't give the feel of a "realtime" scan (with RSSI value, etc updating while scanning).

The drawback to this, is that Core Bluetooth documentation warns:

Disabling this filtering can have an adverse effect on battery life; use it only if necessary.

Kable consumers will need to explicitly set allowDuplicateKeys to false to get the default Core Bluetooth behavior.