Fix GH-7980: Unexpected result for iconv_mime_decode

We need to reset the shift state right after conversion, to cater to
potenially following plain encodings. Also, there is no need to

Fix GH-7980: Unexpected result for iconv_mime_decode

We need to reset the shift state right after conversion, to cater to
potenially following plain encodings. Also, there is no need to reset
the shift for plain encodings, because these are not state-dependent.

Closes GH-8025.

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