Fixup req form data file upload with "r\n" line endings

[MSP-Greg]

Description

Current code breaks when a form uploads a file with "\r\n" line endings.

Currently client.rb uses a string buffer that is initialized as UTF-8, it should be initialized as ASCII-8BIT. UTF-8 can be 'character/line-ending' sensitive, while ASCII-8BIT is just bytes...

Add two tests, both with "\r\n" line endings, one as BOM|UTF-8.

Closes Discussion #3131

Running the two new tests on master has the below errors:

  1) Error:
TestPumaServer#test_form_data_encoding_windows:
NoMethodError: undefined method `split' for nil
    puma/test/test_puma_server.rb:1629:in `test_form_data_encoding_windows'

  2) Error:
TestPumaServer#test_form_data_encoding_windows_bom:
NoMethodError: undefined method `split' for nil
    puma/test/test_puma_server.rb:1598:in `test_form_data_encoding_windows_bom'

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