Cannot skip setting config.mount_at when using mount RailsPerformance::Engine in routes.rb

[LouisVallat]

How to reproduce :

1. Install gem
2. Follow README and put mount RailsPerformance::Engine in routes.rb to a specific route (let's say /dashboard/performance)
3. Comment config.mount_at line in initializer
4. Restart app
5. Rails performance is both mounted at /rails/performance and /dashboard/performance

Looks like to culprit is

rails_performance/lib/rails_performance.rb

Line 61 in 624f829

@@mount_at = "/rails/performance"

as it always sets a default that is then used by

rails_performance/config/routes.rb

Line 21 in 624f829

mount RailsPerformance::Engine => RailsPerformance.mount_at, as: 'rails_performance'

A proposed fix could be to not set the default for @@mount_at or to interpret config.mount_at = false or config.mount_at = nil as a switch to disable auto mounting.

Of course it would also be needed to fix all calls to rails_performance_path.

I could do a pull request to implement either of these solutions if it suits your needs, or let me know if I missed anything 😄

Have a great day