The old function took a string value, which wasn't really correct as
the property type is a GHashTable of string:string.  For whatever
reason this is how nm-applet passed VPN secrets back to NM in the return
from the GetSecrets() D-Bus call.  This was probably easier or
something but it was a special case that's magic and quite unclear.

Since we use nm_connection_update_secrets() more these days, and we
depend on the GValue types we pass into it matching the property
types of the setting property the secret is for, we need to fix that
up for VPN connections.  But keep the old code for backwards
compatibility.

In the future secret agents should pass back VPN secrets in the same
form as the VPN setting specifies them for the "secrets" property:
a GHashTable of string:string.  But the old mechanism of just dumping
the key/value pairs into the returned VPN hash as string:string will
still work.