fix: merge custom data with current one rather than override

[isekovanic]

CLA

• I have signed the Stream CLA (required).
• Code changes are tested

Description of the changes, What, Why and How?

This PR solves an issue where whenever custom data is being passed to an already existing channel, channel.data would be overwritten rather than merged with the custom data. This makes sure that this situation doesn't happen and that the channel is kept in tact regardless of what we passed.

Changelog

[github-actions]

Size Change: +46 B (+0.01%)

Total Size: 427 kB

Filename	Size	Change
dist/browser.es.js	92.6 kB	+8 B (+0.01%)
dist/browser.full-bundle.min.js	54.1 kB	+12 B (+0.02%)
dist/browser.js	93.7 kB	+10 B (+0.01%)
dist/index.es.js	92.6 kB	+7 B (+0.01%)
dist/index.js	93.7 kB	+9 B (+0.01%)

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