Avoid partial name matching

[milanmlft]

Hi @mpadge, thanks for the awesome package!
I just noticed I get some "partial match" warnings when running e.g. get_contributors():

Warning messages:
1: In ctb_code$login : partial match of 'login' to 'logins'
2: In ctb_issues$authors$login : partial match of 'login' to 'logins'
3: In ctb_issues$contributors$login : partial match of 'login' to 'logins'
4: In ctb_issues$authors$login : partial match of 'login' to 'logins'

(Note: I do have options(warnPartialMatchDollar = TRUE, warnPartialMatchAttr = TRUE, warnPartialMatchArgs = TRUE) set locally but I'd argue it's safer to not rely on partial matches 🙂).

This PR should take care of that. Happy to address any feedback or even for you to ignore this if you don't think it's important!