Improve the readability of 'getShippingDetails()' and 'updateDetails' method of UserShippingDetails component

[adityasharma7]

closes #3770

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Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

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IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

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[pkarw]

Please apply the changes suggested by @andrzejewsky in order to merge this

[adityasharma7]

@andrzejewsky @pkarw I have updated the PR with changes suggested by @andrzejewsky. Apart from that, I have also simplified the code for updateDetails() method along with the update to the changelogs.

[andrzejewsky]

@adityasharma7 thanks! @alinadivante please have look at checkout process, request data for shipping details and whether entire placing a order works as beforehand, thanks!