Fix handling of redirect response without Location header

[PikachuEXE]

Fix ArgumentError thrown when the response for the URL does not contain header Location

Got this when trying to use this gem to check and report invalid images from a long list of image URLs

[PikachuEXE]

Not sure why the build is not displayed but it passed:
https://travis-ci.org/github/sdsykes/fastimage/builds/699561478

[sdsykes]

Thanks for the patch, I'll merge it.

BTW, FastImage does not robustly check the validity of images at all.

[PikachuEXE]

I think it's good enough for our case
Just need to catch common issues for images hosted by others (totally not controlled by us)
Then we can shift the blame as long as the issues are reported

[Nakilon]

Not sure how it is "a common issue". Catching redirects for years using my gem built on top of Fastimage and never saw an empty redirect.

[PikachuEXE]

Even it's uncommon I don't think it should throw ArgumentError instead of a meaningful one (which is an "image fetch failure")
I consider the lack of Location header as a form of "bad response"

[PikachuEXE]

@sdsykes Let me know if anything else needed before merging this

[PikachuEXE]

Is this going to be released soon?