Test and fix space-flattening bug.

t/pt-table-checksum/samples/2-space-col.sql

@@ -0,0 +1,8 @@
+drop database if exists test;
+create database test;
+use test;
+create table t (
+  id int auto_increment primary key,
+  `a  b` int not null  -- 2 spaces between a and b
+);
+insert into t values (null, 1),(null, 2),(null, 3),(null, 4),(null, 5),(null, 6),(null, 7),(null, 8),(null, 9),(null, 10);