Fix XML parsing for sites with only one entry

[neizod]

My friend's site using XML feed and only have one entry. When I try to fetch the feed it returns

Trace: TypeError: item.map is not a function
    at parseRSS (file:///home/neizod/feed-reader/src/utils/parser.js:88:24)
    at read (file:///home/neizod/feed-reader/src/main.js:34:27)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
    at async extractFromUrl (file:///home/neizod/feed-reader/eval.js:10:17)
    at extractFromUrl (file:///home/neizod/feed-reader/eval.js:14:13)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)

As I investigate, the item variable, after parsed from XML into JSON, can be an array or an object (like the case of Atom in the same file). Thus, .map function fail to work on when it is actually an object.

This PR should fix the issue.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[coveralls]

Coverage remained the same at 95.122% when pulling f64f48a on neizod:main into 5c4bfc2 on ndaidong:main.

[coveralls]

Coverage remained the same at 95.122% when pulling f64f48a on neizod:main into 5c4bfc2 on ndaidong:main.

[ndaidong]

@neizod thank you for your fix. I will merge this pr now.