fix: test_upload_large_file should open file in binary file mode

[azrle]

Hi @leehambley @mattbrictson

Start working on #483. But I found that the local test dose not pass in my local environment (ruby version: 2.6.6 / macOS).

The test_upload_large_file failed to pass the test due to encoding issue.

...
205 tests, 3381 assertions, 0 failures, 0 errors, 0 skips
...
  test_upload_large_file                                          FAIL (22.06s)
        --- expected
        +++ actual
        @@ -1,3 +1,3 @@
        -# encoding: UTF-8
        - (snipped)
        -#    valid: false
        +# encoding: ASCII-8BIT
        +#    valid: true
        + (snipped)
...
25 tests, 35 assertions, 1 failures, 0 errors, 0 skips

After this fix:

205 tests, 3381 assertions, 0 failures, 0 errors, 0 skips
...
  test_upload_large_file                                          PASS (2.03s)
...
25 tests, 35 assertions, 0 failures, 0 errors, 0 skips

[leehambley]

Excellent catch, thank you for the contribution.