The decimal point '.' is changed to locale's decimal point
before/after JSON conversion to make C standard library's
locale-specific string conversion functions work correctly.
All the tests now call setlocale(LC_ALL, "") on startup to use the
locale set in the environment.
Fixes GH-32.
As of now, the parameter is unused, but may be needed in the future.
I'm adding it now so that in the future both API and ABI remain
backwards compatible as long as possible.
This is a backwards incompatible change.
