test: favor deepStrictEqual over deepEqual
test-http-mutable-headers uses assert.deepEqual() in three places but appears to only needs it in two of them. Replace one with assert.deepStrictEqual() and remove linting exception.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test http