Fix decimal marshalling in specific locales

In certain locales, the decimal separator is a comma instead of a point, so `12.34` becomes `12,34`. This causes an issue with marshalling Decimals; as Pheasant tries to store them as `12,34`. MySQL can't handle this and stores `12.00`.

This PR fixes the issue and adds a testcase. I used the nl_NL in the testcase, but the fixes works for all locales which use a comma as decimal separator (German, Danish, Swedish, French, Italian, etc.)