Our new state variable `@quote_char` was not being initialized
property if you used `ini_subsetting` for a setting that did
not yet exist. This fixes that bug.
Prior to this commit, the `ini_subsetting` type assumed that
all of the settings strings were quoted, and always wrote
out the modified value with double-quotes around it.
This commit adds tests for the case where the original setting
is not quoted, and intelligently writes the modified setting
with the same quote character (or lack thereof) that the
original setting used.