fix: #1868 Clone node, Setting className for SVG element raises error

[hal1984]

Summary

This PR fixes/implements the following bugs/features

• Clone node: Setting className for SVG element raises error #1868

Explain the motivation for making this change. What existing problem does the pull request solve?

See #1868. The definition of className differs for Element and SVGElement. In Element className it is a string, but in SVG element class name it is a instance of SVGAnimatedString.

References: https://developer.mozilla.org/en-US/docs/Web/API/Element/className
https://developer.mozilla.org/en-US/docs/Web/API/SVGAnimatedString

Code formatting

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Closing issues

fixes #1868
closes #1868

[niklasvh]

Thanks for the PR! I did a small update to it in #2079

[JohnJ255]

Same for SVGSVGElement

html2canvas v1.0.0-rc.5
Uncaught (in promise) TypeError: Cannot assign to read only property 'className' of object '#<SVGSVGElement>'
at DocumentCloner.resolvePseudoContent (document-cloner.ts:412)
at DocumentCloner.cloneNode (document-cloner.ts:274)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at DocumentCloner.cloneNode (document-cloner.ts:284)
at new DocumentCloner (document-cloner.ts:54)
at index.ts:81
at step (tslib.es6.js:97)
at Object.next (tslib.es6.js:78)
at tslib.es6.js:71
at new Promise ()
at __awaiter (tslib.es6.js:67)
at renderElement (index.ts:30)
at html2canvas (index.ts:23)