Issue with Path Params Support in reverse_proxy_plug

[YevhenKysilov]

I am very excited about your project and was looking forward to integrating reverse_proxy_plug into my work project. During the integration process, I noticed that the plug seems to lack support for path parameters in the upstream URL. Specifically, path variables are not replaced with their corresponding values, leading to incorrect request routing.

Code Example:
forward "/users/:userId", to: ReverseProxyPlug, upstream: "https://example.com/users/:userId"

In this example, I expected :userId to be replaced by the actual user ID from the request, but this substitution does not occur.

Here is the solution to this issue
#174

[mwhitworth]

This is a good idea. I'll take a look and incorporate your changes.