fix #2609 fix deepMerge with date type

[bluebill1049]

fix #2609

[codesandbox-ci]

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Sandbox	Source
react-hook-form/react-hook-form: app	Configuration
objective-poitras-sms37	Issue #2609
nervous-ride-f7x7t	Issue #2609

[github-actions]

Size Change: +57 B (0%)

Total Size: 91.7 kB

Filename	Size	Change
dist/index.cjs.development.js	14.6 kB	+10 B (0%)
dist/index.cjs.production.min.js	8.62 kB	+6 B (0%)
dist/index.esm.js	14.6 kB	+10 B (0%)
dist/index.ie11.development.js	18.8 kB	+8 B (0%)
dist/index.ie11.production.min.js	11.2 kB	+6 B (0%)
dist/index.umd.development.js	14.9 kB	+11 B (0%)
dist/index.umd.production.min.js	8.69 kB	+6 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
dist/index.ie11.js	143 B	0 B
dist/index.js	142 B	0 B

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