addHeader may cause error

[liumapp]

in some times , filter may do it's job several times .

which will cause error like this :

use setheader instead of add , will solve this problem

[mohitsoni]

@liumapp Interesting situation.

I'm curious to understand how we got into this particular situation. Is it because:

1. CORSFilter got invoked multiple times in one request/response cycle ?
   1.1 Is this because CORSFilter is configured multiple times in the filter chain ?
2. There's another filter in your filter chain that duplicates the CORSFilter functionality ?

The change seems fine to me, but I would like to understand the context better.

[liumapp]

We have configured two filters, but we make sure the CORS's filter are executed first.

The initial guess may be related to Shiro.

But my amendment can solve this problem .

I hope you are willing to accept my pull request