TRUNK-3625: Fixed web image handler

org.openmrs.web.controller.observation.handler.WebImageHandler#getObs
must pass valid java.util.Locale object to org.openmrs.Obs#getValueAsString.
Locale is now obtained from Context.

See https://issues.openmrs.org/browse/TRUNK-3625
Merge pull request #971 from vencik/master

TRUNK-3625: Fixed web image handler