Improve TzIf format detection

[n-vr]

Added the four-octet ASCII sequence to identify TzIf files: 0x54 0x5A 0x69 0x66
Source

This pull request fixes #214

[coveralls]

Coverage remained the same at 96.407% when pulling 64aff2c on n-vr:improve-tzif-detection into 272584c on gabriel-vasile:master.

[gabriel-vasile]

Hi, thank you for taking your time to contribute.

The problem I tried to describe in #214 is that the detection is too loose. Any simple text file that happens to start with TZif is detected as application/tzif (see the commit I added on this PR, it shows the problem.)

https://tools.ietf.org/id/draft-murchison-tzdist-tzif-00.html#rfc.section.3 describes in detail the format of the tzif file. For example, fifth byte, version, must be 0x00, 0x32 or 0x33, charcnt bytes must not be 0, etc.
These are the checks that we can do for tzif files to make the detection more accurate.

[n-vr]

Hello @gabriel-vasile,

Thanks for creating a test case. I did not fully understand the problem, and I think I do now. I created some additional checks:

• File length at least 44 bytes (length of full header)
• Version MUST be 0x00, 0x32 or 0x33
• typecnt MUST not be zero

[n-vr]

Sorry for overwriting twice, my signature was not working properly because of a new system install.

[gabriel-vasile]

Hi, code looks better but there are still some issues to fix. I left some suggestions.

You can commit how many times you want, there is not problem.

[gabriel-vasile]

Thank you for your contribution!