RFR: 8263552: Use String.valueOf() for char-to-String conversion in ObjectStreamClass

[Andrey Turbanov]

On Sat, 20 Feb 2021 12:17:32 GMT, Сергей Цыпанов <github.com+10835776+stsypanov at openjdk.org> wrote:

> This is a very simple and trivial improvement about getting rid of pointless char wrapping into array

Marked as reviewed by turbanoff at github.com (no known OpenJDK username).

I think it's worth to cleanup other places with similar code too.
I found 6 places in JDK codebase:
![изображение](https://user-images.githubusercontent.com/741251/109499351-d6fc8d00-7aa5-11eb-9306-a70cb6754d55.png)

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PR: https://git.openjdk.java.net/jdk/pull/2660