force binary encoding in a ruby2.1-compatible way

tests/acceptance/proxy_tests.rb

@@ -4,7 +4,7 @@ require 'flexnbd/fake_dest'
 
 module ProxyTests
   def b
-    String.new("\xFF", encoding: "BINARY")
+    "\xFF".b
   end
 
   def with_proxied_client( override_size = nil )

tests/acceptance/test_serve_mode.rb

@@ -6,7 +6,7 @@ class TestServeMode < Test::Unit::TestCase
 
   def setup
     super
-    @b = String.new("\xFF", encoding: "BINARY")
+    @b = "\xFF".b
     @env = Environment.new
     @env.writefile1( "0" )
     @env.serve1