function defaultMixinMergeStrategy (mergeObject, mixinObject) {
  return Object.assign(mixinObject, mergeObject)
}
Default strategy for creating `mergeObject` from arguments and the result from `mixin()`.
Fields from `mergeObject` have higher priority in this strategy.

@param {Object} mergeObject The object a user has supplied to the logging function.
@param {Object} mixinObject The result of the `mixin` method.
@return {Object}