Add PrecompiledAssets::Manifest#inspect

[NiklasHae]

Sometimes an exception leads to printing the inspected manifest object. This fills multiple screens with the parsed manifest hash. Most of the times though, the exception's message Could not find "foobar.css" in manifest: <PrecompiledAssets::Manifest ....> already helps to solve the problem.

Instead of relying on the #inpect default implementation it truncates @parsed_manifest.

Feel free to decline without comment if you don't want this change merged ;) Thx 👍

[foobear]

Thanks for your suggestion. A useful inspection does make sense.
However, I think there is room for improvement here.

• Convention for object inspection is to start with #<, not <#.
• If the manifest has not yet been parsed this inspects with @parsed_manifest=... instead of @pared_manifest=....
• I'm not sure how helpful the first 8 characters of the manifest.json can be. Maybe we should show more characters, or offer some kind of aggregation, like @parsed_manifest=(23 entries).
• The object_id is not actually put directly into inspection by default, but implementation varies across Ruby versions, so maybe don't want to go there. I'm fine with keeping it as implemented, or not displaying it at all.

With a few adjustments, we can get an inspection like

#<PrecompiledAssets::Manifest:12345 @pathname=#<Pathname:/path/to/manifest.json> @parsed_manifest={"application.js"=>"application-5ZACKM24...>

or, if the manifest has not been parsed,

#<PrecompiledAssets::Manifest:12345 @pathname=#<Pathname:/path/to/manifest.json> @parsed_manifest=nil>

Suggested change

	truncated_parsed_manifest = "#{@parsed_manifest.to_s[0..7]}..."
	"<#{self.class.name}:#{object_id} @pathname=#{@pathname.inspect} @parsed_manifest=#{truncated_parsed_manifest}>"
	manifest_inspection = @parsed_manifest.inspect.sub(/\A(.{40}).+\z/, '\1...')
	"#<{self.class.name}:#{object_id} @pathname=#{@pathname.inspect} @parsed_manifest=#{manifest_inspection}>"

[NiklasHae]

• Good catch
• 👍
• I like the @parsed_manifest=(23 entries) suggestion 👍
• I've been here https://bugs.ruby-lang.org/issues/17199 before, and I just used object_id :D. A little bit weird for Ruby to not have a straightforward way to override #inspect.

Since I'm running a patched version anyways, I'll look into adapting this PR with your suggestions.