create blob only 'content' is required

[gemal]

https://github.com/KnpLabs/php-github-api/blob/master/lib/Github/Api/GitData/Blobs.php#L62

requires both content and encoding

but according to the documentation:
https://docs.github.com/en/rest/reference/git#create-a-blob

only content is required

[acrobat]

@gemal You are right! Can you send a PR to remove the encoding key from the check? Thanks!