Do no unescaped already unescaped bytea values

If the unescaped value contains a null byte, pg 0.18 will raise an
exception.

activerecord/lib/active_record/connection_adapters/postgresql/oid.rb

@@ -27,6 +27,9 @@ def type_cast(value)
         class Bytea < Type
           def type_cast(value)
             return if value.nil?
+            # This is a flawed heuristic, but it avoids truncation;
+            # we really shouldn’t be calling this with already-unescaped values
+            return value if value =~ /\x00/
             PGconn.unescape_bytea value
           end
         end

activerecord/test/cases/dirty_test.rb

@@ -4,6 +4,7 @@
 require 'models/parrot'
 require 'models/person'   # For optimistic locking
 require 'models/aircraft'
+require 'models/binary'
 
 class Pirate # Just reopening it, not defining it
   attr_accessor :detected_changes_in_after_update # Boolean for if changes are detected
@@ -261,6 +262,14 @@ def test_float_zero_to_string_zero_not_marked_as_changed
     assert_empty data.changes
   end
 
+  def test_binary_with_null_byte_with_same_value_not_marked_as_changed
+    binary = Binary.create!(data: "\\\\foo\x00\\\\bar")
+
+    binary.data = "\\\\foo\x00\\\\bar"
+
+    assert_not binary.changed?, "should be unchanged"
+  end
+
   def test_zero_to_blank_marked_as_changed
     pirate = Pirate.new
     pirate.catchphrase = "Yarrrr, me hearties"