az network vnet-gateway list-advertised-routes has incomplete data

**Describe the bug**
Executing az network vnet-gateway list-advertised-routes lists routes, but does not appear to correctly populate 'origin' or 'sourcePeer' for routes learned from other connections.

**To Reproduce**
Have a VPN Gateway with 2 or more BGP enabled VPN connections, run:

`az network vnet-gateway list-advertised-routes -g -n --peer --output json`

For routes learned from other connections you will see similar to:

{
"asPath": "65525-65020",
"localAddress": "10.200.8.30",
"network": "10.5.32.0/20",
"nextHop": "10.200.8.30",
"origin": "Incomplete",
"sourcePeer": null,
"weight": 0
}

**Expected behavior**
Expected sample should show the correct details of the route, for example:

{
"asPath": "65525-65020",
"localAddress": "10.200.8.30",
"network": "10.5.32.0/20",
"nextHop": "10.200.8.30",
"origin": "EBgp",
"sourcePeer": "10.4.28.253",
"weight": 0
}

**Environment summary**
Installed via apt-get on Ubuntu on WSL on Windows 10 Pro 1809 17763.316
azure-cli 2.0.58
network 2.3.2

**Additional context**
N/A