Mutating _rev on localDocument in conflict resolver results in merge error

resolveDocMerge extracts the previous local rev from the mutable conflict.localDocument.  However, this may have been modified by the conflict resolver.  As a result, conflict resolvers that return a modified localDocument (stripping out _rev) will result in an error like:

Error resolving conflict for <ud>mydoc</ud>: Attempted to tombstone active revision for doc (mydoc), but provided rev () doesn't match current rev(14-f9841ac5d3bfa186fbd6f52d1f747cd8)
Any code that works with Conflict after conflict resolution has run should treat it as mutated, and only use when appropriate (rev check for local/remote wins, and body for merge)