Fixes issue with process method not visible in integration tests

Since `process` was private in `ActionDispatch::Integration::RequestHelpers::Session`, when you call `process` inside an integration test it would never call `process` at the current session as Ruby 2.0 and onwards only checks for public methods and `process` being private is always a false result when `method_missing` gets called inside `ActionDispatch::Integration::RequestHelpers::Runner`.

Making it public allows it to be called again inside integration tests and fixes rspec/rspec-rails#925.