the date parser shouldn't use the system locale

Right now the date parser we use in AbstractDateJIRAFieldDisplayer doesn't have a specified locale, so it relies on the system locale.
This can be a problem if the system locale is different from the info sent by JIRA.
This is already immediately visible when running the integration tests on a system using a different locale than english: the integration are currently failing.

Apparently the XML returned by JIRA expose a language field: we could use it as a source of the locale for the parser, but we should check first that a JIRA configured to not use english would then provide a different format of dates in the returned result (it's my expectation, but to be tested). If that's the case then, this is a real bug.

If it's not the case, then we should probably always force the locale to be in english.