[WIP] Use mb_convert_encoding if it's available

[takuseno]

This is the PR copied from here.

Make Charset use mb_convert_encoding if the function is available.

This motivation comes from the issue that iconv is not working correctly on Alpine Linux.

https://bugs.alpinelinux.org/issues/5025

[coveralls]

Coverage decreased (-0.3%) to 96.979% when pulling 5c6888c on takuseno:master into 9e98fd1 on php-mime-mail-parser:master.

[coveralls]

Coverage decreased (-0.3%) to 96.979% when pulling 5c6888c on takuseno:master into 9e98fd1 on php-mime-mail-parser:master.

[coveralls]

Coverage decreased (-0.3%) to 96.979% when pulling 0fd22f3 on takuseno:master into 9e98fd1 on php-mime-mail-parser:master.

[coveralls]

Coverage decreased (-0.3%) to 96.979% when pulling 0fd22f3 on takuseno:master into 9e98fd1 on php-mime-mail-parser:master.

[eXorus]

@takuseno Hello, thanks for your contribution. It is always work in progress ?

[takuseno]

@eXorus
Hi, sorry that it's still in progress. I'm trying to disable mb_convert_encoding by runkit.

[eXorus]

@takuseno Hello, any news about this ? are you still in progress ? it will be great to have it :)

[takuseno]

@eXorus
Sorry for replying late. I didn't have enough time to make testings. Probably we have to use runkit or apd to disable mb_convert.