fix: fix issue #1325 and add missing truncated field in the GHGist object

[SCHJonathan]

Description

1. Fixes #1325 by adding a lazy download from the raw_url field of GHGistFile object if the gist file content is uninitialized.
2. From the list public gist documentation, I found out we are missing the truncated field in the GHGist object and I added that field in this PR.

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[bitwiseman]

Thanks for contributing.

Sorry about all the comments. This is basically a good PR with test coverage an everything, but you ran it in a bunch of the gotchas of working in the library all at once.

I'm happy to answer any questions you might have about the comments.

[SCHJonathan]

@bitwiseman. Thank you so much for your help and detailed comment, bitwiseman! All of those suggestions are super helpful and I have made revisions based on your suggestions!

[bitwiseman]

A few more bits of feedback.

[bitwiseman]

Sorry, why add this?

[SCHJonathan]

Sorry, why add this?

Sorry, the reason I am adding this setter is that I want to pass the owner object from the GHGist object to each GHGistFile object so that get content can use owner.root() to create request. At a second thought, probably defining a wrapUp method to set the owner as the GHGist object owning this current GHGistFile object would make more sense. I have make this revision accordingly. Thank you again for your suggestions.

[bitwiseman]

@SCHJonathan
You're very close on this. Do you time to finish this PR in the next few days? I plan to do a release soon.

[bitwiseman]

It looks like some test is failing.