Entry.WithFields(): copy the original entry

[Julio-Guerra]

Entry.WithFields() currently creates a new Entry and partially copies it. Calling several times WithFields() on an already logged entry (e.g. in a formatter) thus leads to an incorrectly initialized Entry structure.

The patch makes the method copy the original entry and only modify the Data member with the new set of fields. So that calling several times WithFields() on an already logged entry (e.g. in a formatter Format() method) avoids this problem.

[DmitriyMV]

We should probably fix #529 first because merging this will cause race detector to go nuts.