When a query parameter has no value, output is as `key=`

[mildm8nnered]

When a URL query parameter has no value, AWS expects it to be presented as key=. processURL did not add the = sign.

See https://github.com/soto-project/soto-core/issues/523

[adam-fowler]

From a quick look the change looks fine. I'm on holiday just now. Will review properly when I return in 10 days time.

[codecov[bot]]

Codecov Report

Merging #524 (853e8e3) into main (f8a8db6) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #524   +/-   ##
=======================================
  Coverage   72.49%   72.49%           
=======================================
  Files          78       78           
  Lines        6134     6134           
=======================================
  Hits         4447     4447           
  Misses       1687     1687           

Impacted Files	Coverage Δ
Sources/SotoSignerV4/signer.swift	90.00% <100.00%> (ø)

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