(maint) Fix valid_url? check

Because the 'request' function does NOT return the value the block you pass it returns, this function was always returning a Net::HTTP object. With how this function was being used, it would result in valid_url? always being truthy.  Net::HTTP.start DOES return the value that the block you pass it returns, though, so this simply un-blockifies the processing of the request response.

lib/vanagon/component/source/http.rb

@@ -22,20 +22,19 @@ def valid_url?(target_url)
             return false unless ['http', 'https'].include? uri.scheme
 
             Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == 'https') do |http|
-              http.request(Net::HTTP::Head.new(uri)) do |response|
-                case response
-                when Net::HTTPRedirection
-                  # By parsing the location header, we get either an absolute
-                  # URI or a URI with a relative `path`. Adding it to `uri`
-                  # should correctly update the relative `path` or overwrite
-                  # the entire URI if it's absolute.
-                  location = URI.parse(response.header['location'])
-                  valid_url?(uri + location)
-                when Net::HTTPSuccess
-                  return true
-                else
-                  false
-                end
+              response = http.request(Net::HTTP::Head.new(uri))
+              case response
+              when Net::HTTPRedirection
+                # By parsing the location header, we get either an absolute
+                # URI or a URI with a relative `path`. Adding it to `uri`
+                # should correctly update the relative `path` or overwrite
+                # the entire URI if it's absolute.
+                location = URI.parse(response.header['location'])
+                valid_url?(uri + location)
+              when Net::HTTPSuccess
+                true
+              else
+                false
               end
             end
           end