Check for nil response in db#command and set check_response for filemd5.

Kyle can we make check_response default to true instead of false?

lib/mongo/db.rb

@@ -463,7 +463,7 @@ module Mongo
       result = Cursor.new(system_command_collection, :admin => admin,
         :limit => -1, :selector => selector, :socket => sock).next_document
 
-      if check_response && !ok?(result)
+      if result.nil? || (check_response && !ok?(result))
         raise OperationFailure, "Database command '#{selector.keys.first}' failed: #{result.inspect}"
       else
         result

lib/mongo/gridfs/grid_io.rb

@@ -340,7 +340,7 @@ module Mongo
       md5_command            = BSON::OrderedHash.new
       md5_command['filemd5'] = @files_id
       md5_command['root']    = @fs_name
-      @server_md5 = @files.db.command(md5_command)['md5']
+      @server_md5 = @files.db.command(md5_command, false, true)['md5']
       if @safe
         @client_md5 = @local_md5.hexdigest
         if @local_md5 != @server_md5