(TK-43) Fix query param issue with proxy

This PR fixes the issue reported by @praxxis wherein a proxy handler added with add-proxy-route would crash when given a query parameter.
Seems like another alternative for this change would be to just have uri be initialized as a `java.lang.StringBuilder` instead of being implicitly initialized by `str` to a `java.lang.String`.  That way, the `.append` method that the original code was trying to reference would actually make sense, since `append()` is available for a `StringBuilder`.  This would change the uri definition to something like:

```clj
uri (StringBuilder. (str scheme "://" ...))
```