Indent output properly when dumping values

This fixes the indentation logic when dumping values, which was producing
misinx results with way too much indentation in some cases. Also, it adds
unit tests for the "wcfg.dumps()" API and checks for proper indentation of
the output.
Fix encoding of numeric values when dumping

Properly encode numeric values to UTF-8/ASCII when writing them to the
output stream. This also addds unit test for "wcfg._dump_value()".