THREESCALE-8426 - request path is stripped for proxied https requests

[samugi]

This is a fix for THREESCALE-8426 where in some situations the client's request path is stripped away from the proxied upstream request.

Steps to reproduce the issue:

• Backend with public path = /
• Upstream with https:// scheme and a path in its private URL, like: https://echo-api.3scale.net/some/path
• Https proxy (configured on APIcast for upstream connections)
• Send a request to APIcast with a certain path appended http://public-base-url/foo/bar
• Verify the request was actually sent to https://echo-api.3scale.net/some/path instead of the expected https://echo-api.3scale.net/some/path/foo/bar

Problem

The problem is that we set the upstream request's path using uri.path (when available) here.

uri.path, is upstream.uri.path, where upstream.uri is self.uri i.e. only the prefix (aka the upstream path that is hardcoded in the Private URL when the Backend is defined), which explains the problem as described above.

Fix

Currently we rewrite ngx.var.uri to contain the exact path for the upstream request here by appending the value of ngx.var.uri to the Backend prefix.
This means we should use ngx.var.uri as the path for the proxied request here, instead of uri.path.

[kevprice83]

Looks good, added a new test to cover the additional issue found (same problem occurs when routing policy is enabled)