fix(fabric.Gradient): Guard against deep mutation on svg export for color exports

[asturur]

This was an edge case, but since the fix is actually trivial and in a non hot code path, is still worth fixing.

[github-actions]

Code Coverage Summary

> fabric@5.1.0 coverage:report
> nyc report --reporter=lcov --reporter=text

-----------|---------|----------|---------|---------|-----------------------------------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                             
-----------|---------|----------|---------|---------|-----------------------------------------------
All files  |   82.05 |    74.69 |   84.44 |   80.67 |                                               
 fabric.js |   82.05 |    74.69 |   84.44 |   80.67 | ...-27584,27700-27701,27722-27763,27778-27937 
-----------|---------|----------|---------|---------|-----------------------------------------------

[asturur]

The bug was added in the migration, but a test was missing anyway.
The reason for the bug was the removal of a deep clone statement.

[ShaMan123]

Was there a deep clone statement? I think this bug existed regardless

[asturur]

https://github.com/fabricjs/fabric.js/pull/8001/files#diff-2e1e3a2e5dc880cba81870b75734070b74d9282630f4b85b3f923999045fc110L222

wasn't the migration indeed, was before, i compared with v5.x and i assumed all the chages were from the ts migration