Coherent behaviour for NaN and Infinity inputs (uncurrency filtering)

[leodido]

Uncurrency class should:

• handle NaN values
  • return double(NAN) for input string representing values that are not a number, also if they are localised (e.g. "ليس رقم")
  • locale 'ar_AE', uncurrencying "ليس رقم" outputs double(NAN)
  • locale 'it_IT0, uncurrencying "NaN" outputs double(NAN)
  • inputs like "NaN €" are always considered invalid (and so returned as they are) because
  • a currency without a number can not exist
  • $fmt->format(NAN) (where $fmt is a \NumberFormatter intl instance) never returns an accordingly formatted string but only a (possibly localised) NaN value
• handle infinity values
  • currency is mandatory: uncurrencying "∞ €" outputs double(INF) (i.e. filtered)
  • currency is NOT mandatory: uncurrencying "∞" outputs "∞" (i.e. not filtered)