Fix tar headers with a 101 character name

[ptomulik]

Description:

This attempts to fix #1608. The algorithm used by Gem::Package::TarWritter#split_name is revised and changed such that for names longer than 100 bytes (when split is required) it:

• guarantees that non-empty name is returned,
• guarantees that non-empty prefix is returned (if we split name into new name and prefix, and prefix appears to be empty, then the result is incorrect as we put the new name plus no prefix to TarHeader but the new name differs from the original one, so the resultant path is incorrect - this happens to the current implementation of split_name found in master, for example when the path is 101-characters long and starts with "/"),
• tries to push as much of the name to prefix as possible (but not more than 155 bytes) - this is how the GNU tar works, at least from my observation,

The PR consists initially of two commits, first with tests only (to show, where the old implementation fails) and second containing the new implementation of #split_name.

Tasks:

• Describe the problem / feature
• Write tests
• Write code to solve the problem
• Get code review from coworkers / friends
• Squash commits

I will abide by the code of conduct.

[segiddins]

This looks good to me, especially since it has such good test coverage 👍

[ptomulik]

Thanks.