Handle overwriting functions

[goliatone]

Currently if we have a function as an attribute of one of the objects to be overwriting we end up extending the function instead of replacing it.

const defaults = {
 //We want to shim console
  logger: _=> console
};

const config = {
  logger: {level: 'info', log(){}...}
};
let info = extend({}, defaults, config);
typeof info.logger === 'function'
typeof info.logger.level === 'string'

Make sure we compare to funct:

if (source[property] && source[property].constructor &&
   source[property].constructor === Object) {
  target[property] = target[property] || {};
  if (typeof target[property] === 'function') target[property] = source[property];//<<< ADD
  else target[property] = extend(target[property], source[property]);
} else target[property] = source[property];