The TP-DCS field in TPDUs specified in GSM 03.40 and following GSM 23.040
paragraph 9.2.3.10 can take values defined in GSM 03.38 and following
23.038 paragraph 4.
The latter divides the octet into the first four bits which indicate the
coding group used for the PDU.

Mistakenly the 'isBinary()' function of org.ajwcc.pduUtils.gsm3040.Pdu
assumes the message to be purely binary if its coding group is set to
'1111', although it is possible for messages to transport text even though
they indicate using said coding group which will result in them being
classified as binary messages although these messages can be processes and
their text can be decoded correctly.

I do not have a definite fix for this yet, but in order to mitigate the
issue I took advantage of the fact those text messages appear to always be
of 7 bit enconding while real binary messages appear to be of 8 bit
enconding which can be used to distinguish between the two types.
Explicitly I would like to make clear that at this point I am not sure if
there can be text messages of said coding group with 8 bit enconding which
would at least partially break the fix.

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