bridge.login_shared_secret_map is wiped on bridge start

[aine-etke]

When trying to use the new appservice double-puppeting method, the value of bridge.login_shared_secret_map.<server_name> is replaced with '' (empty string) on bridge start.

Before the start (note that quotation doesn't matter, the same happens when value is quoted with '', "", and without quotes):

    login_shared_secret_map:
        <server_name>: 'as_token:<uuid-v4>'

After the start:

    login_shared_secret_map:
        <server_name>: ''

Bridge's version: 75c3bcb