Fix broken spec due to EXPLAIN EXTENDED on MySQL 8

The behavior that used to be triggered by EXPLAIN EXTENDED has been the
default for a long time and the previously deprecated EXTENDED keyword
was finally removed in MySQL 8.0.

To avoid breaking the spec on very old MySQL versions we just catch the
SQL syntax error and retry without the EXTENDED keyword.

Author: felixbuenemann

spec/mysql2/client_spec.rb

@@ -232,7 +232,15 @@ def connect *args
       it "should > 0" do
         # "the statement produces extra information that can be viewed by issuing a SHOW WARNINGS"
         # http://dev.mysql.com/doc/refman/5.0/en/explain-extended.html
-        @client.query("explain extended select 1")
+        begin
+          @client.query("explain extended select 1")
+        rescue Mysql2::Error
+          # EXTENDED keyword is deprecated in MySQL 8.0 and triggers a syntax error
+          # https://dev.mysql.com/doc/refman/5.7/en/explain-extended.html
+          # "extended output is now enabled by default, so the EXTENDED keyword is superfluous and
+          # deprecated ... and it will be removed from EXPLAIN syntax in a future MySQL release"
+          @client.query("explain select 1")
+        end
         @client.warning_count.should > 0
       end
     end