minor: make DB#validate_collection compatible with server v1.8 and v1.9.1

lib/mongo/db.rb

@@ -602,7 +602,7 @@ module Mongo
       if !ok?(doc)
         raise MongoDBError, "Error with validate command: #{doc.inspect}"
       end
-      if !doc['valid']
+      if (doc.has_key?('valid') && !doc['valid']) || (doc['result'] =~ /\b(exception|corrupt)\b/i)
         raise MongoDBError, "Error: invalid collection #{name}: #{doc.inspect}"
       end
       doc

test/db_test.rb

@@ -304,7 +304,11 @@ class DBTest < Test::Unit::TestCase
 
     should "validate collection" do
       doc = @db.validate_collection(@coll.name)
-      assert doc['valid']
+      if @@version >= "1.9.1"
+        assert doc['valid']
+      else
+        assert doc['result']
+      end
     end
 
   end