Make tests pass with umask

[ghost]

Assertions on file permissions only pass with some combinations of
file create mode and the process file mode creation mask. This mask
should be applied on the file create mode before related assertions.

I needed to raise the permissions allowed by my file mode creation
mask to make the tests on File#atomic_write pass. It fix these tests
for any umask value.

[ghost]

Here is an extract of a dump (some parts removed) from standard output
when tests are failing:

% cd activesupport
% rake test
[...]
Finished tests in 4.649657s, 552.9441 tests/s, 1789.5945 assertions/s.

  1) Failure:
test_atomic_write_preserves_default_file_permissions(AtomicWriteTest) [.../rails/activesupport/test/core_ext/file_test.rb:54]:
Expected: 33161
  Actual: 33152

  2) Failure:
test_atomic_write_preserves_file_permissions(AtomicWriteTest) [.../rails/activesupport/test/core_ext/file_test.rb:33]:
Expected: 33261
  Actual: 33216

2571 tests, 8321 assertions, 2 failures, 0 errors, 0 skips
[...]