Agent::Connected is only called for the first bridge

[nicklaswj]

Problem
The function Agent::Connected for public agents is only called for the first connected bridge,
i.e. subsequent bridge connections does not result in calls to Agent::Connected.

Environment:

• Yew version [e.g. v0.14, master]
• stdweb / web-sys

I got two slightly different solutions: #1029 and #1030