fix guard on riak_kv_w_reduce:validate_arg for dialyzer
fix guard on riak_kv_w_reduce:validate_arg for dialyzer

riak_kv_w_reduce.erl:224: The variable Fun can never match since previous clauses completely covered the type fun()

Whether or not Fun is a function is checked in the call to validate_fun.
The guard that was removed here was preventing the proper error message
from being returned.

andytill/riak_kv