The problem is, if user explicitly specifies a collation or a
charset name for a column, which is same as server default
collation, then the SHOW CREATE TABLE command does not show them.
If the output of SHOW command is executed on a server with
different default collation, then the column would be assigned
with collation same as server default collation, and the
collation that was explicitly specified by user originally is
lost.

The collation is either specified explicitly by user in CREATE
TABLE command or it is implicitly assigned based on current
server defaults. The server stores the collation, but it does not
have information if the collation was explicitly specified by
user or was picked-up implicitly. SHOW CREATE TABLE command just
ignores showing the collation information if the collation is
same as server default character name or collation.

The expectation is that the SHOW CREATE TABLE command should
always show the collation if user originally did explicitly
specify the collation. This patch stores a boolean flag in new
dictionary column mysql.columns.is_explicit_collation indicating if
collation was explicitly provided by user. SHOW CREATE TABLE uses
this information to always show collation information if user
specified it explicitly. SHOW CREATE TABLE always displays both
character set name and collation for a table, if user has
explicitly provided one of character set name or the collation
name.

A test case is added in main.show_check.

Change-Id: I424c0977b5d51db13f7298ee312d07920ef0d191