Make process method safer by coercing path into a string

Currently the `process` method in `Capybara::RackTest::Browser` takes an argument called `path` and assumes it is a string as it treats it as such by using it as an argument to `URI.parse` and sending it methods like `start_with?`.

This patch coerces the path argument with `to_s`.

@twalpole I thought this a more appropriate near term fix for my desired long term fix by tackling the issue at the driver level.