Extend specs of IO#pwrite

[herwinw]

No description provided.

[andrykonchin]

suggestion: there is another case that makes sense to cover.

When #to_s returns non-String value - then it's ignored and object is still converted to String with rb_any_to_s CRuby function:

VALUE
rb_any_to_s(VALUE obj)
{
    VALUE str;
    VALUE cname = rb_class_name(CLASS_OF(obj));

    str = rb_sprintf("#<%"PRIsVALUE":%p>", cname, (void*)obj);

    return str;
}

that prints object in the following way:

"#<Object:0x0000000106095428>"

[herwinw]

I'm not sure if this is something we need to test here, if we add it here it should also be added to IO#print, IO#write, IO#syswrite and all other IO methods I'm forgetting. But this code is unrelated to IO, so I guess this should be tested in object/to_s_spec.rb or something else that's more generic.

[andrykonchin]

Thank you!