change uri parser regexp due to uri gem change

From ruby/uri#107:

```
This pull-request switched URI::DEFAULT_PARSER to URI::RFC3986_Parser. And I added switch-back feature for URI::RFC2396_Parser.

URI.parser = URI::RFC2396_PARSER
```

lib/logstash/inputs/http_poller.rb

@@ -155,7 +155,7 @@ def normalize_request(url_or_spec)
   def validate_request!(url_or_spec, request)
     method, url, spec = request
 
-    raise LogStash::ConfigurationError, "Invalid URL #{url}" unless URI::DEFAULT_PARSER.regexp[:ABS_URI].match(url)
+    raise LogStash::ConfigurationError, "Invalid URL #{url}" unless URI::RFC2396_PARSER.regexp[:ABS_URI].match(url)
 
     raise LogStash::ConfigurationError, "No URL provided for request! #{url_or_spec}" unless url
     if spec && spec[:auth]