[sr-dev] [kamailio/kamailio] {s.encode.hexa} transformation does not work as expected (#2690)

We have found an issue in version 5.2 To reproduce it

``` $var(x) = "wg48D8NPIJ5JEfkgqhuu+Q=="; $var(decode64) = $(var(x){s.decode.base64}); $var(encode16) = $(var(decode64){s.encode.hexa}); ```

after these line the value of encode16 must be 'c20e3c0fc34f209e4911f920aa1baef9'

Looking inside the module pv, file pv_trans.c, line 347, we realized that the code is not correct because the shift operation keeps the sign,

`_tr_buffer[j++] = fourbits2char[val->rs.s[i] >> 4];`

we saw that if we modifies the previous line with the following

`_tr_buffer[j++] = fourbits2char[(unsigned char)(val->rs.s[i]) >> 4];`

the function works as expected