parser: fix the select field text [DNM]

[tiancaiamao]

Try to fix the select field text bug introduced in #587

mysql> SELECT @@`autocommit` LIMIT 1;
+------------------------+
| @@`autocommit` LIMIT 1 |
+------------------------+
| 1                      |
+------------------------+
1 row in set (0.00 sec)

which should be

mysql> SELECT @@`autocommit`;
+----------------+
| @@`autocommit` |
+----------------+
| 1              |
+----------------+
1 row in set (0.00 sec)

I'm not sure it's a good fix, pr/587 seems to introduce some other problems.
For example, is this a correct gramma in MySQL?

select 1 where true group by 1 having 1>0

[codecov]

Codecov Report

Merging #596 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #596      +/-   ##
==========================================
+ Coverage   79.75%   79.76%   +<.01%     
==========================================
  Files          32       32              
  Lines       12489    12493       +4     
==========================================
+ Hits         9961     9965       +4     
  Misses       1939     1939              
  Partials      589      589

Impacted Files	Coverage Δ
parser.go	92.68% <100%> (ø)	⬆️

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