Fix Duplicate Headers

[MuckT]

• Changed the headersFieldNamesToLowerCase method to handle duplicate headers taking the last value provided.
• Updated tests to reflect changes.
• Likely fixes: Failed to convert header keys to lower case due to field name conflict: content-type #2235

This is my first PR to nock please let me know if there are any additional steps needed.

[mastermatt]

Hey thanks for the PR. It looks like this was work that was already done but never got merged in (then closed accidentally).
The previous thread with discussion on approach is in #1969 and the resulting PR is #1972
I'd prefer we move forward with 1972 instead of this PR as it maintains the desired behavior of still erroring on dup headers in Nock options.

[MuckT]

Closing in favor of #1972.