Fix the regex pattern to reduce backtracking

This should fix #1832.

app/models/agents/rss_agent.rb

@@ -206,7 +206,7 @@ module Agents
       else
         # Encoding is already known, so do not let the parser detect
         # it from the XML declaration in the content.
-        body.sub!(/(\A\u{FEFF}?\s*<\?xml(?:\s+\w+\s*=\s*(['"]).*?\2)*)\s+encoding\s*=\s*(['"]).*?\3/, '\\1')
+        body.sub!(/(?<noenc>\A\u{FEFF}?\s*<\?xml(?:\s+\w+(?<av>\s*=\s*(?:'[^']*'|"[^"]*")))*?)\s+encoding\g<av>/, '\\k<noenc>')
       end
       body
     end