Fix retrieval of get_post_table_characterset (#231)

Query command run through `assoc_args_to_str` get double quotes truncated and fail to execute. 

Using single quotes in the query prevent error.

`Warning: Failed to get current character set of the posts table. Reason: ERROR at line 1: Unknown command '\_'`

Error may be more prominent on windows.

src/DB_Command.php

@@ -657,8 +657,8 @@ private function get_posts_table_charset( $assoc_args ) {
  . 'FROM information_schema.`TABLES` T, '
  . 'information_schema.`COLLATION_CHARACTER_SET_APPLICABILITY` CCSA '
  . 'WHERE CCSA.collation_name = T.table_collation '
- . 'AND T.table_schema = "' . DB_NAME . '" '
- . 'AND T.table_name LIKE "%\_posts";';
+ . "AND T.table_schema = '" . DB_NAME . "' "
+ . "AND T.table_name LIKE '%\_posts';";
 
  list( $stdout, $stderr, $exit_code ) = self::run(
  sprintf(