Fix mishandling of SASL attribute parsing

[gwynne]

When parsing SASL attribute strings, we were incorrectly allowing them to be split on = characters more than once per attribute. This broke for attribute values ending with =, most particularly Base64-like values, which appear often in - for example - the nonce strings sent by Supabase PostgreSQL severs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6ce96ab) 59.92% compared to head (339b9d1) 62.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
==========================================
+ Coverage   59.92%   62.20%   +2.28%     
==========================================
  Files         124      124              
  Lines        9937     9936       -1     
==========================================
+ Hits         5955     6181     +226     
+ Misses       3982     3755     -227     

Files	Coverage Δ
...IO/Utilities/SASLAuthentication+SCRAM-SHA256.swift	47.48% <100.00%> (+47.21%)	⬆️

... and 5 files with indirect coverage changes

[fabianfett]

Two tiny nits! Extremely good work! Thanks so much! Love the increase in test coverage as well!