Make `seems?/2` always return a boolean

[warmwaffles]

Predicate methods should always return a boolean. Mixing true | false | nil is a bit inconsistent.

[rNoz]

Hey @warmwaffles thanks for this.

While I understand the criteria, the idea of returning nil was some sort of "unsupported and/or not recognized format", in case other formats are provided, while false indicates purely that the binary it is not that image format.

However, if no objections, I'll accept it in a couple of days ;)

[warmwaffles]

Heh, the docs don't specify that nil means it is unsupported though. Just says "false otherwise".

https://github.com/Group4Layers/ex_image_info/blob/e9627da5990413c291fb66134adf40ac330f9e34/lib/ex_image_info.ex#L82-L86

Not a huge deal, just didn't realize that it was intentional. My use case is just checking if the image file type is supported in our application. Which we would coerce the nil to false using ==

@spec supported(String.t()) :: boolean()
def supported?(path) do
  case File.read(path) do
    {:ok, content} -> ExImageInfo.seems?(content) == true
    _ -> false
  end
end

[rNoz]

Sorry, in that case I missed that in the docs. Do you mind we keep the original behavior? In that case, maybe you can update the docs in this PR ;)

[warmwaffles]

Yea I can open another PR with the docs rectified. There are other places where I can see nil is being used.