Default `IO::Buffer#get_string` to use BINARY encoding.

io_buffer.c

@@ -1125,7 +1125,7 @@ io_buffer_get_string(int argc, VALUE *argv, VALUE self)
 
     size_t offset = 0;
     size_t length = data->size;
-    rb_encoding *encoding = NULL;
+    rb_encoding *encoding = rb_ascii8bit_encoding();
 
     if (argc >= 1) {
         offset = NUM2SIZET(argv[0]);

test/ruby/test_io_buffer.rb

@@ -71,7 +71,10 @@ class TestIOBuffer < Test::Unit::TestCase
 
   def test_file_mapped
     buffer = File.open(__FILE__) {|file| IO::Buffer.map(file)}
-    assert_include buffer.get_string, "Hello World"
+    contents = buffer.get_string
+
+    assert_include contents, "Hello World"
+    assert_equal Encoding::BINARY, contents.encoding
   end
 
   def test_file_mapped_invalid