Don't send 103 early hints response when only invalid headers are used

[MSP-Greg]

Description

While reviewing code in test\test_response_header.rb, I noticed that when invalid headers are returned by the app with 'early hints', the first line of an 'early hints' response (HTTP/1.1 103 Early Hints) is sent without any data/headers.

This PR modifies the code in puma/request.rb to only send the 'early hints' response if at least one header is valid.

Note that with the test changes, current master has the following errors:

TestResponseHeader#test_illegal_character_in_key_when_early_hints [test/test_response_header.rb:128]:
Expected # encoding: ASCII-8BIT
#    valid: true
"HTTP/1.1 103 Early Hints\r\n\r\n" to not include "HTTP/1.1 103 Early Hints".

  2) Failure:
TestResponseHeader#test_illegal_character_in_value_when_early_hints [test_response_header.rb:139]:
Expected # encoding: ASCII-8BIT
#    valid: true
"HTTP/1.1 103 Early Hints\r\n\r\nHTTP/1.0 200 OK\r\nContent-Length: 5\r\n\r\nHello" to not include "HTTP/1.1 103 Early Hints".

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.

[nateberkopec]

this double negative is messing with me, but I can't see a better alternative