Instead of blindly assuming that we can take whatever string given as
valid UTF-8, we'll always attempt to convert from the current modem
charset into UTF-8. Before we were doing this for hex-encoded UCS2,
but not for example for GSM-7.
And due to the now applied GSM-7 conversion, the mf627a/mf627b +COPS
parsing unit tests are updated accordingly, because when converting
from an input string that contains byte 0x40 ('@' in UTF-8) as if it
were GSM-7, the 0x40 is taken as character '¡', encoded as 0xc2,0xa1
in UTF-8).
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