fixes setting array syntax

[zzantozz]

Existing handling of setting array indexes has a couple of issues. First, it calls a non-existent lists:append/3 in a couple of places. It also mishandles setting the first element of an array greater than length one. This adds tests that cover the problem spots in the code and fixes the problems.

Problem behavior before this change can be seen in

> props:set("a[1].b", 2, {[{<<"a">>, [{[{<<"b">>, 1}]}]}]}).
** exception error: undefined function lists:append/3
     in function  props:do_set/3 (/vagrant/lib/props/src/props.erl, line 176)

and

> props:set("a[1]", 2, {[{<<"a">>, [4,5,6]}]}).
{[{<<"a">>,[2,6]}]}