Array#+ sends to_ary to non-array arguments

Thanks to removing the assertion in W_ArrayObject.method_add RubySpec's
core/array/plus_spec.rb can be now executed. 6/8 examples pass.
Array#+ coerces the argument to an array
Missing return in Coerce.array
Use array's listview(space) instead of items_w

Building bin/topaz works again.
Simplify Coerce.array: don't create a new array
Coerce.array returns space.listview(w_obj)
Coerce.array: call listview on the result of convert_type
Array#+: test array coercion using #to_ary
Merge pull request #320 from jstepien/array_+

Array#+ sends to_ary to non-array arguments