Fix #930 union join bug, Add test for union join.

[jverswijver]

in 0.13.2 we are improperly forming the sql query when we perform a union and then a join, below are the make_sql()'s comparing 0.13.2 to 0.12.9:

(q1 + q2).make_sql() #0.13.2

(SELECT DISTINCT `table_a`,`table_b` FROM `test`.`table_b` WHERE(table_a < 3)) 
UNION 
(SELECT DISTINCT `table_a`,`table_b` FROM `test`.`table_b` WHERE(table_a > 3))

((q1 + q2) * TableA).make_sql() #0.13.2

SELECT DISTINCT `table_a`,`table_b` 
FROM (
    SELECT DISTINCT `table_a`,`table_b` 
        FROM `test`.`table_b` 
        WHERE(table_a < 3)) as `$8` 
NATURAL JOIN (
    SELECT DISTINCT `table_a`,`table_b` FROM `test`.`table_b` 
    WHERE(table_a > 3)
    ) as `$9`

(q1 + q2).make_sql() #0.12.9

SELECT `table_a`,`table_b` 
FROM (
    SELECT `table_a`,`table_b` 
    FROM `test`.`table_b` WHERE (table_a < 3) 
    UNION 
    SELECT `table_a`,`table_b` 
    FROM `test`.`table_b` 
    WHERE (table_a > 3)
    ) as `_u4`

((q1 + q2) * TableA).make_sql() #12.9

SELECT `table_a`,`table_b` 
FROM (
    SELECT `table_a`,`table_b` 
    FROM `test`.`table_b` 
    WHERE (table_a < 3) 
    UNION 
    SELECT `table_a`,`table_b` 
    FROM `test`.`table_b` 
    WHERE (table_a > 3)
    ) as `_u5` 
NATURAL JOIN `test`.`table_a`

Below is the make_sql after I added code to fix it

((q1 + q2) * TableA).make_sql() #0.13.2 DEV

SELECT DISTINCT `table_a`,`table_b` 
FROM (
    (SELECT DISTINCT `table_a`,`table_b` FROM `test`.`table_b` WHERE(table_a < 3)) 
    UNION 
    (SELECT DISTINCT `table_a`,`table_b` FROM `test`.`table_b` WHERE(table_a > 3))
    ) as `$8` 
NATURAL JOIN 
(SELECT DISTINCT `table_a` FROM `test`.`table_a`) as `$9`

I think this solution is good but I have included all the make_sql()'s to ensure that my solution did not introduce any unintended consequences.

fix #930

[guzman-raphael]

@jverswijver 🎉 Woohoo! Nice work with this contribution! 🎉