Fix regression introduced by ee61bc3f making it impossible to merge 2 operations.

ee61bc3f added this : https://framagit.org/kresusapp/kresus/blob/master/server/models/operation.js#L198 The function does not have the appropriate signature leading to an error:

[2019-01-19T16:34:07.018] [ERROR] helpers - when merging two operations: Cannot read property 'dateImport' of undefined
[2] [2019-01-19T16:34:07.020] [ERROR] HTTP - PUT /api/v1/operations/78ecab481d1540fb92e9820b560cfb9d/mergeWith/51eb1d6dde964be19676eddbc9bdf319 - 500 

This is the error returnes by the server:

{"code":null,"message":"Cannot read property 'dateImport' of undefined"}