fix warnings by ineffassign, with a one actual issue

In store/search.go, we would make a copy of a byte array, but then still use
the original instead of the copy. Could result in search operations not finding
messages that do have the content, but under very unlikely conditions only.

We'll keep running ineffassign with "make check", useful enough.

store/search.go

@@ -187,7 +187,7 @@ func toLower(buf []byte) []byte {
 			copied = true
 			nr := make([]byte, len(r), len(r)+nsize+len(buf)-i)
 			copy(nr, r)
-			nr = r
+			r = nr
 		}
 		r = utf8.AppendRune(r, nc)
 	}