Bug#11757005: UNION CONVERTS UNSIGNED MEDIUMINT AND BIGINT

              TO SIGNED
Problem:
When we are joining types (of fields) in case of a union, we usually
upgrade the datatypes to the largest present in the query.
In case of mediumint, it is not happening.
Analysis:
When joined with types LONG and LONGLONG, mediumint should get
upgraded to LONG and LONGLONG respectively.
W.r.t the given query, constant '1' will be created as a LONGLONG
internally and SIGNED flag is enabled. As a result, while combining
types for the field, LONGLONG along with MEDIUMINT gets converted
to LONG first. LONG with MEDIUMINT(of the third select) gets converted
to MEDIUMINT. SIGNED FLAG would be that of the first field's.
As a result, the final result would be SIGNED MEDIUMINT.
Fix:
While joining types, MEDIUMINT with LONGLONG and MEDIUMINT with LONG
is converted to LONGLONG and LONG respectively. Also, made some 
changes for FLOAT and DOUBLE.

sql/field.cc

@@ -184,7 +184,7 @@ static enum_field_types field_types_merge_rules [FIELDTYPE_NUM][FIELDTYPE_NUM]=
   //MYSQL_TYPE_NULL         MYSQL_TYPE_TIMESTAMP
     MYSQL_TYPE_LONG,         MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_LONGLONG     MYSQL_TYPE_INT24
-    MYSQL_TYPE_LONGLONG,    MYSQL_TYPE_INT24,
+    MYSQL_TYPE_LONGLONG,    MYSQL_TYPE_LONG,
   //MYSQL_TYPE_DATE         MYSQL_TYPE_TIME
     MYSQL_TYPE_VARCHAR,     MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_DATETIME     MYSQL_TYPE_YEAR
@@ -215,7 +215,7 @@ static enum_field_types field_types_merge_rules [FIELDTYPE_NUM][FIELDTYPE_NUM]=
   //MYSQL_TYPE_NULL         MYSQL_TYPE_TIMESTAMP
     MYSQL_TYPE_FLOAT,       MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_LONGLONG     MYSQL_TYPE_INT24
-    MYSQL_TYPE_FLOAT,       MYSQL_TYPE_INT24,
+    MYSQL_TYPE_FLOAT,       MYSQL_TYPE_FLOAT,
   //MYSQL_TYPE_DATE         MYSQL_TYPE_TIME
     MYSQL_TYPE_VARCHAR,     MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_DATETIME     MYSQL_TYPE_YEAR
@@ -246,7 +246,7 @@ static enum_field_types field_types_merge_rules [FIELDTYPE_NUM][FIELDTYPE_NUM]=
   //MYSQL_TYPE_NULL         MYSQL_TYPE_TIMESTAMP
     MYSQL_TYPE_DOUBLE,      MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_LONGLONG     MYSQL_TYPE_INT24
-    MYSQL_TYPE_DOUBLE,      MYSQL_TYPE_INT24,
+    MYSQL_TYPE_DOUBLE,      MYSQL_TYPE_DOUBLE,
   //MYSQL_TYPE_DATE         MYSQL_TYPE_TIME
     MYSQL_TYPE_VARCHAR,     MYSQL_TYPE_VARCHAR,
   //MYSQL_TYPE_DATETIME     MYSQL_TYPE_YEAR
@@ -277,7 +277,7 @@ static enum_field_types field_types_merge_rules [FIELDTYPE_NUM][FIELDTYPE_NUM]=
   //MYSQL_TYPE_NULL         MYSQL_TYPE_TIMESTAMP
     MYSQL_TYPE_NULL,        MYSQL_TYPE_TIMESTAMP,
   //MYSQL_TYPE_LONGLONG     MYSQL_TYPE_INT24
-    MYSQL_TYPE_LONGLONG,    MYSQL_TYPE_INT24,
+    MYSQL_TYPE_LONGLONG,    MYSQL_TYPE_LONGLONG,
   //MYSQL_TYPE_DATE         MYSQL_TYPE_TIME
     MYSQL_TYPE_NEWDATE,     MYSQL_TYPE_TIME,
   //MYSQL_TYPE_DATETIME     MYSQL_TYPE_YEAR