fix: use custom merger for options to avoid array values concatenation

[Qrzy]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

As pointed in #78, defu (both used here and in Nuxt's code) was the reason of even triplicated values in the headers.
This PR aims fixing that by applying 2 changes:

• removing defaults from module definition - so that Nuxt does not apply regular defu on defaults and options from the module user
• creating custom merger with createDefu that in case of arrays replaces the value instead of concatenating them

Fixes #78

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

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Name	Status	Preview	Updated
nuxt-security	✅ Ready (Inspect)	Visit Preview	Dec 29, 2022 at 9:45AM (UTC)

[Baroshem]

Works great! Merging.