Fixed Catalan translation (lowercase for months and days of the week)

Months and days of the week should be in lowercase (see references and justification below).
Changed both source and test code.

- Catalan Wikipedia: 
  - http://ca.wikipedia.org/wiki/Mes
  - http://ca.wikipedia.org/wiki/Setmana
- Style Book, Pompeu Fabra University, Barcelona: http://www.upf.edu/leupf/07esc/65_3.htm
- Guidelines, Barcelona University: http://www.ub.edu/slc/ffll/fitxes/Fitxa26.html