Fixed Thrift::TypeError exception in specs

An error occurs while running specs, that changes the behavior of the system under test, leading to false positive:

   NonblockingServer
   Thrift::NonblockingServer
   #<Thread:0x0000ffff67b9f138 /code/lib/rb/spec/nonblocking_server_spec.rb:147 run> terminated with exception (report_on_exception is true):
   /code/lib/rb/lib/thrift/types.rb:69:in 'Thrift.check_type': Expected Types::DOUBLE, received Integer for field seconds (Thrift::TypeError)
          from /code/lib/rb/lib/thrift/struct.rb:157:in 'block in SpecNamespace::NonblockingService::Sleep_args#field_accessor'
          from /code/lib/rb/lib/thrift/client.rb:41:in 'block in Thrift::Client#send_message_args'
          from /code/lib/rb/lib/thrift/client.rb:40:in 'Hash#each'
          from /code/lib/rb/lib/thrift/client.rb:40:in 'Thrift::Client#send_message_args'
          from /code/lib/rb/lib/thrift/client.rb:30:in 'Thrift::Client#send_message'
          from /code/lib/rb/spec/gen-rb/nonblocking_service.rb:75:in 'SpecNamespace::NonblockingService::Client#send_sleep'
          from /code/lib/rb/spec/gen-rb/nonblocking_service.rb:70:in 'SpecNamespace::NonblockingService::Client#sleep'
          from /code/lib/rb/spec/nonblocking_server_spec.rb:160:in 'block in RSpec::ExampleGroups::NonblockingServer::ThriftNonblockingServer#setup_client_thread'
   should kill active messages when they don't expire while shutting down

When an integer 10 is sent to "sleep", instead of actually sleeping an error happens, and the method never called, leading to no record of the method execution recorded in the result. Which is the same expectation is if the method call was aborted in the middle.