LiveData - block assigning reserved keys

[leandrocp]

Beacon.Content.create_assign_for_live_data/2 can't allow creating an assign for the following reserved keys:

• :beacon
• [:uploads, :streams, :socket, :myself] (https://github.com/phoenixframework/phoenix_live_view/blob/376fea7ba6a655d5bafe47f0698a672d3ad12f62/lib/phoenix_component.ex#L556)
• :flash (https://github.com/phoenixframework/phoenix_live_view/blob/376fea7ba6a655d5bafe47f0698a672d3ad12f62/lib/phoenix_component.ex#L1317)