Give the configuration context precedence over context in integrations

[imjoehaines]

Goal

If Configuration#context is set then it will now take precedence over the context set by integrations. This is true even if it is set to nil, in which case no context will be sent by the notifier — the context will be set by the Bugsnag app instead

For example:

The top error has config.context = nil, so the context is set by the Bugsnag app The middle error has config.context = 'some context' and so always uses that value as the context The bottom error has no context set in config, which allows the Rack integration's context to be used