Fix replacement logic in replace_server_args (ensure replacement does not use capture indexes)

[yezhiyi9670]

When using unfiltered string as replacement, it is important to escape the % characters to prevent "invalid capture index" errors. This is properly done in:

• htaccess-for-nginx/htaccess.lua

  Line 1290 in ec2de6a

  dst = dst:gsub('%$'..i, match:gsub('%%', '%%%%')..'') -- make sure no capture indexes are being used as replacement

• htaccess-for-nginx/htaccess.lua

  Line 1296 in ec2de6a

  dst = dst:gsub('%%'..i, match:gsub('%%', '%%%%')..'') -- make sure no capture indexes are being used as replacement

But not here:

• htaccess-for-nginx/htaccess.lua

  Line 809 in ec2de6a

  result = result:gsub('%%{'..org_svar..'}', replace)..''

Consider this rewrite rule set, which is modified from Wordpress default:

<IfModule mod_rewrite.c>
RewriteEngine On
# RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php?%{QUERY_STRING} [L]
</IfModule>

If the last rule is triggered with query string that contains something like %2C, the script will crash. This commit fixes it by also doing replace:gsub('%%', '%%%%')..''.

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The second commit fixed #37. I accidentally committed it to the same branch.