RUBY-187 ByteBuffer should take another ByteBuffer as initial data

lib/bson/byte_buffer.rb

@@ -23,15 +23,19 @@ module BSON
     attr_reader :order
 
     def initialize(initial_data="")
-      if initial_data.is_a?(String)
+      @str = case initial_data
+        when String then
           if initial_data.respond_to?(:force_encoding)
-          @str = initial_data.force_encoding('binary')
+            initial_data.force_encoding('binary')
           else
-          @str = initial_data
+            initial_data
           end
+        when BSON::ByteBuffer then
+          initial_data.to_a.pac('C*')
         else
-        @str = initial_data.pack('C*')
+          initial_data.pack('C*')
       end
+
       @cursor = @str.length
       @order  = :little_endian
       @int_pack_order    = 'V'

test/bson/bson_test.rb

@@ -74,6 +74,11 @@ class BSONTest < Test::Unit::TestCase
     end
   end
 
+  def test_round_trip
+    doc = {'doc' => 123}
+    @encoder.deserialize(@encoder.serialize(doc))
+  end
+
   # In 1.8 we test that other string encodings raise an exception.
   # In 1.9 we test that they get auto-converted.
   if RUBY_VERSION < '1.9'