fix String::GetValue memory leaking and suport assignment of const char*

[guyingzhao]

1. change delete to free since plist_get_string_val allocate memory with malloc
2. support assigning String by const char* to avoid memory duplication

[nikias]

There are conflicts. Could you please try to not change the coding style/indentation? That makes it easier for me. I already redid your earlier two commits so we just have the changes. Thanks.

[guyingzhao]

@nikias I have resolved the conflicts and submitted the code again, but it seems that the branch have some issues on CI checks. I don't think the errors come from my code and it works fine locally. Please let me know if I need to fix my code, thanks.

Well, I dived into the error logs and fixed them, the branch is clean to be merged. Please have a look at this pull request @nikias

[guyingzhao]

@nikias , could you please review the request so I can go back to original libplist repo.

[nikias]

@guyingzhao ok I think I addressed almost all of your changes now.

[guyingzhao]

@guyingzhao ok I think I addressed almost all of your changes now.

Thanks a lot, I'll create another request to fix the assert error under debug mode.