[BUG] Combination of I/O proxies and config image spec

[jreichel-nvidia]

... = OIIO::ImageInput::open(extension, nullptr, ioproxy);

works fine. But adding a config spec fails:

OIIO::ImageSpec config;
config["oiio:UnassociatedAlpha"] = 1;
... = OIIO::ImageInput::open(extension, &config, ioproxy);

create() in imageinput.cpp:115 succeeds (and sets the proxy), but open() in imageinput.cpp:119 tries to obtain the proxy again from the config (why?), which does not contain it.

I can set the proxy additionally on the config via

config.attribute("oiio:ioproxy", OIIO::TypeDesc::PTR, &ioproxy);

but this is a bit redundant and cumbersome, in particular figuring out the syntax for that and the "&ioproxy" vs "ioproxy" part. Tested with BMP and GIF.