fix(string_stream): add missing initializations of const format specifiers

[KnoerleMaTLS]

This pull request introduces a possible fix for the issue #848 by adding the missing initializers.

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[jwellbelove]

Were you getting warning from the compiler?

[KnoerleMaTLS]

Warning about what/where? Regarding the propsed fix?

[jwellbelove]

Did you get errors or warnings about 'missing initializers'? left_spec & right_spec have default constructors, so they shouldn't need explicit initialisation.

[KnoerleMaTLS]

Yeah, i got compiler errors about missing initializers for left and right. Please check #848 for further details.

Yes, the structs left_spec and right_spec have compiler generated default constructors, but regarding the C++ standard this is not enough to initialize a const object ("default-initialization of a const object could not call an implicitly declared default constructor"). I also tried to explain this in more detail in the mentioned issue.