Use URI::DEFAULT_PARSER

…also re-enables RSpec warnings.

This change addresses the warning around `URI.extract` being obsolete.
The method still exists, but seems like new(ish) Rubies prefer use of
`URI::DEFAULT_PARSER` (or one of the other parsers).

Author: jgarber623

lib/webmention/parsers/plaintext_parser.rb

@@ -10,7 +10,7 @@ class PlaintextParser < Parser
 
       # @return [Array<String>] An array of absolute URLs.
       def results
-        @results ||= URI.extract(response_body, %w[http https])
+        @results ||= URI::DEFAULT_PARSER.extract(response_body, %w[http https])
       end
     end
   end

spec/spec_helper.rb

@@ -79,7 +79,7 @@
 
   # This setting enables warnings. It's recommended, but in some cases may
   # be too noisy due to issues in dependencies.
-  # config.warnings = true
+  config.warnings = true
 
   # Many RSpec users commonly either run the entire suite or an individual
   # file, and it's useful to allow more verbose output when running an