Ensure original size is included if any widths are larger and !allowUpscale

[JaredReisinger]

Change Image.getValidWidths() logic to ensure original width is included if any larger-than-original widths are requested in the !allowUpscale case.

Added test to cover this scenario.

Fixes #184

Note that the implementation here is slightly different from the one mentioned in #184; I didn't like the overly-complicated map() logic with it all crammed in the first line, and it's important that the string->int parsing in line 186 happens before we attempt to compare widths. Plus, adding an additional map() call—but only when !allowUpscale—is balanced by the removal of the now-not-needed filter(), and we also no longer need to special-case the "we had valid widths, but filtered removed them all, so we need to push the original size back into the list" scenario.

From a "what do you expect" standpoint, there are cases where you ask for a specific list of widths (like [300, 1200]), and you might get a width not on that list ([300, 840]). This already happens, though, when upscaling is disabled and you ask only for widths larger than the original. This effectively enables the list of widths to include both a thumbnail width and a maximum width at the same time, ensuring that images narrower than the maximum don't result in only the thumbnail size. (This is useful in photo gallery/lightbox scenarios.)

[zachleat]

I will merge this but I think I’m going to add an additional threshold so that we don’t get [300, 310] width outputs. It needs to be bigger than the previous size by a certain threshold to be added.

[zachleat]

Shipping with v3.2.0

[zachleat]

This will be the minimumThreshold option and the default will be 1.25 to start.

When the original width is smaller than the desired output width, this is the minimum size difference
between the next smallest image width that will generate one extra width in the output.
e.g. when using widths: [400, 800], the source image would need to be at least (400 * 1.25 =) 500px wide
to generate two outputs (400px, 500px). If the source image is less than 500px, only one output will
be generated (400px).