webRequest block no longer confuses specified url vs url from path

Fixes #535 - when specifying a UrlPath, the variable 'url' was being set to the string found in an incoming message at that path. This caused a couple problems:

1. the rule window in the streamtools UI would show values in both UrlPath and Url 
2. streamtools would error: "Specify either a url or a path to a url"
3. subsequent messages with new urls would just trigger a request to the first url.

Ack. This fixes all of those problems. I tested a few different scenarios:

1. Url set -> change to UrlPath -> change back to Url
2. UrlPath set -> change to Url -> back to UrlPath
3. UrlPath set -> send different urls in json -> change to Url

etc.