Use C objectid_generate for ObjectId in addition to ObjectID.

Also, remove broken defined? check from objectid.rb and object_id.rb -
it was never doing anything since we require those files at the top of
cbson.c.

ext/cbson/cbson.c

@@ -949,6 +949,7 @@ void Init_cbson() {
     DigestMD5 = rb_const_get(Digest, rb_intern("MD5"));
 
     rb_define_method(ObjectID, "generate", objectid_generate, 0);
+    rb_define_method(ObjectId, "generate", objectid_generate, 0);
 
     rb_define_method(rb_cArray, "fast_pack", fast_pack, 0);
 }

lib/bson/types/object_id.rb

@@ -155,8 +155,7 @@ module BSON
 
     private
 
-    # We need to define this method only if CBson isn't loaded.
-    unless defined? CBson
+    # This gets overwritten by the C extension if it loads.
     def generate
       oid = ''
 
@@ -175,7 +174,6 @@ module BSON
 
       oid.unpack("C12")
     end
-    end
 
     def get_inc
       @@lock.synchronize do

lib/bson/types/objectid.rb

@@ -158,8 +158,7 @@ module BSON
 
     private
 
-    # We need to define this method only if CBson isn't loaded.
-    unless defined? CBson
+    # This gets overwritten by the C extension if it loads.
     def generate
       oid = ''
 
@@ -178,7 +177,6 @@ module BSON
 
       oid.unpack("C12")
     end
-    end
 
     def get_inc
       @@lock.synchronize do