integer being interpreted as a character code when assigning to a string

integer being interpreted as a character code when assigning to a string

clang-tidy original warning:

`src/cls/rgw/cls_rgw.cc: warning: an integer is interpreted as a character code when assigning it to a string; if this is intended, cast the integer to the appropriate character type; if you want a string representation, use the appropriate conversion facility [bugprone-string-integer-assignment]
key = BI_PREFIX_CHAR;
^
/home/suyash/ceph/src/cls/rgw/cls_rgw.cc:51:24: note: expanded from macro 'BI_PREFIX_CHAR' #define BI_PREFIX_CHAR 0x80`

**On the following lines we are getting the warning:**

1. 138
2. 319
3. 342
4. 2875
5. 2966
6. 3294
7. 3304
8. 3307
9. 3418
10. 3421

Signed-off-by: Suyash Dongre <suyashd999@gmail.com>