function mergeClasses(a,b) {
  if (!a && !b) return '';
  if (!a) return b;
  if (!b) return a;
  if (isArray(a)) a = a.join(' ');
  if (isArray(b)) b = b.join(' ');
  return a + ' ' + b;
}
@ngdoc function
@name pascalprecht.translate.$translateSanitizationProvider#removeStrategy
@methodOf pascalprecht.translate.$translateSanitizationProvider

@description
Removes a sanitization strategy from the list of known strategies.

@param {string} strategyName - unique key for a strategy
@returns {object} this