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

@@ -155,26 +155,24 @@ module BSON
 
     private
 
-    # We need to define this method only if CBson isn't loaded.
-    unless defined? CBson
-      def generate
-        oid = ''
+    # This gets overwritten by the C extension if it loads.
+    def generate
+      oid = ''
 
-        # 4 bytes current time
-        time = Time.new.to_i
-        oid += [time].pack("N")
+      # 4 bytes current time
+      time = Time.new.to_i
+      oid += [time].pack("N")
 
-        # 3 bytes machine
-        oid += Digest::MD5.digest(Socket.gethostname)[0, 3]
+      # 3 bytes machine
+      oid += Digest::MD5.digest(Socket.gethostname)[0, 3]
 
-        # 2 bytes pid
-        oid += [Process.pid % 0xFFFF].pack("n")
+      # 2 bytes pid
+      oid += [Process.pid % 0xFFFF].pack("n")
 
-        # 3 bytes inc
-        oid += [get_inc].pack("N")[1, 3]
+      # 3 bytes inc
+      oid += [get_inc].pack("N")[1, 3]
 
-        oid.unpack("C12")
-      end
+      oid.unpack("C12")
     end
 
     def get_inc

lib/bson/types/objectid.rb

@@ -158,26 +158,24 @@ module BSON
 
     private
 
-    # We need to define this method only if CBson isn't loaded.
-    unless defined? CBson
-      def generate
-        oid = ''
+    # This gets overwritten by the C extension if it loads.
+    def generate
+      oid = ''
 
-        # 4 bytes current time
-        time = Time.new.to_i
-        oid += [time].pack("N")
+      # 4 bytes current time
+      time = Time.new.to_i
+      oid += [time].pack("N")
 
-        # 3 bytes machine
-        oid += Digest::MD5.digest(Socket.gethostname)[0, 3]
+      # 3 bytes machine
+      oid += Digest::MD5.digest(Socket.gethostname)[0, 3]
 
-        # 2 bytes pid
-        oid += [Process.pid % 0xFFFF].pack("n")
+      # 2 bytes pid
+      oid += [Process.pid % 0xFFFF].pack("n")
 
-        # 3 bytes inc
-        oid += [get_inc].pack("N")[1, 3]
+      # 3 bytes inc
+      oid += [get_inc].pack("N")[1, 3]
 
-        oid.unpack("C12")
-      end
+      oid.unpack("C12")
     end
 
     def get_inc