Handle robots.txt on splay rpc service

[vschiavoni]

If a bot passes over the listening service of the Splay RPC, it crashes:

[2011-09-29 20:38:48] ERROR `/robots.txt' not found.
66.249.71.139 - - [29/Sep/2011:20:38:48 CEST] "GET /robots.txt HTTP/1.1" 404 293
- -> /robots.txt
[2011-09-29 20:38:48] ERROR RuntimeError: A JSON Object must begin with '{'
       /var/lib/gems/1.8/gems/ruby-json-1.1.2/./json/objects.rb:172:in
`from_json'
       /var/lib/gems/1.8/gems/Orbjson-0.0.4/lib/orbjson.rb:160:in `process'
       /var/lib/gems/1.8/gems/Orbjson-0.0.4/lib/orbjson.rb:193:in
`process_request'
       /var/lib/gems/1.8/gems/Orbjson-0.0.4/lib/orbjson.rb:199:in `do_GET'
       /usr/lib/ruby/1.8/webrick/httpservlet/abstract.rb:35:in `__send__'
       /usr/lib/ruby/1.8/webrick/httpservlet/abstract.rb:35:in `service'
       /usr/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
       /usr/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
       /usr/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
       /usr/lib/ruby/1.8/webrick/server.rb:162:in `start'
       /usr/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
       /usr/lib/ruby/1.8/webrick/server.rb:95:in `start'
       /usr/lib/ruby/1.8/webrick/server.rb:92:in `each'
       /usr/lib/ruby/1.8/webrick/server.rb:92:in `start'
       /usr/lib/ruby/1.8/webrick/server.rb:23:in `start'
       /usr/lib/ruby/1.8/webrick/server.rb:82:in `start'
       cli-server.rb:80
66.249.71.139 - - [29/Sep/2011:20:38:48 CEST] "GET /json-rpc HTTP/1.1" 500 326
- -> /json-rpc

It happens if the RPC service is started on classic HTTP ports.