fix(proxy-backend): prefix routes with / if not present in config

This commit ensures that the default `pathRewrite` configuration for a proxy element is more resilient to the combination of whether:
- `route` is prefixed with `/`
- `pathPrefix` on the plugin is suffixed with `/` (which at present it will never be - see comments).

Signed-off-by: Brian Fox <brianhfox@gmail.com>

docs/plugins/proxying.md

@@ -36,7 +36,7 @@ Example:
 ```yaml
 # in app-config.yaml
 proxy:
-  '/simple-example': http://simple.example.com:8080
+  simple-example: http://simple.example.com:8080
   '/larger-example/v1':
     target: http://larger.example.com:8080/svc.v1
     headers:
@@ -46,10 +46,11 @@ proxy:
 ```
 
 Each key under the proxy configuration entry is a route to match, below the
-prefix that the proxy plugin is mounted on. It must start with a slash. For
-example, if the backend mounts the proxy plugin as `/proxy`, the above
-configuration will lead to the proxy acting on backend requests to
-`/api/proxy/simple-example/...` and `/api/proxy/larger-example/v1/...`.
+prefix that the proxy plugin is mounted on. If it does not start with a slash,
+one will be prefixed automatically. For example, if the backend mounts the proxy
+plugin as `/proxy`, the above configuration will lead to the proxy acting on
+backend requests to `/api/proxy/simple-example/...` and
+`/api/proxy/larger-example/v1/...`.
 
 The value inside each route is either a simple URL string, or an object on the
 format accepted by