fix(parse_dsn) fatal string concat with nil error

Author: pintsized

lib/resty/redis/connector.lua

@@ -147,7 +147,7 @@ local function parse_dsn(params)
 
         local m, err = ngx_re_match(url, url_pattern, "oj")
         if not m then
-            return nil, "could not parse DSN: " .. err
+            return nil, "could not parse DSN: " .. tostring(err)
         end
 
         local fields
@@ -172,6 +172,7 @@ local function parse_dsn(params)
 
     return true, nil
 end
+_M.parse_dsn = parse_dsn
 
 
 function _M.connect(self, params)