Fix spaces removal on digests with control chars

[JavierJF]

Right now queries with control characters can result in digests containing multiple spaces:

query:

mysql_query(proxy, "SELECT\t\t\t\t\t\t\t\t1");

digest:

mysql> select digest_text,count_star from stats_mysql_query_digest_reset;
+-----------------+------------+
| digest_text     | count_star |
+-----------------+------------+
| SELECT        ? | 1          |
+-----------------+------------+
1 row in set (0.01 sec)

This is undesired behavior, whenever multiple characters that are classified as an space character (see is_space_char function in c_tokenizer.cpp) are found in sequence, they should be compressed into one single space.

[renecannao]

A suggestion for different approach: both *(shared_st->res_cur_pos-1) and shared_st->prev_char should already be ' ' if they were space chars .
Maybe the different approach will make absolutely no difference in term of performance