bug: cors plugin return repeated cross-domain header

[Donghui0]

Issue description

When the backend also returns cross-domain headers, using cors plugin will cause duplicate cross-domain headers
for example:
backend code set header:

enable cors plugin, then response header like this :

Environment

• apisix version (cmd: apisix version): 1.2
• OS: centos7.6

What's the expected result?

remove duplicate headers

[membphis]

If we want to remove the duplicate header, should we keep the upstream or the newly set value of the plugin?

[moonming]

@ShiningRush please take a look. Myabe we can overwrite the headers from upstream?

[ShiningRush]

I think we can add a parameter to control if overwrite the headers of upstream, such as "is_overwrite_upstream".
@membphis @moonming How about it ?

[moonming]

default value is false or ture for is_overwrite_upstream?

[ShiningRush]

I think false is better, upstream should be high priority

[ShiningRush]

After I talk with @membphis , I remove the option of is_overwrite_upstream, because we want plugin as simple as possible.
Now the plugin will overwrite upstream header defaultly.

[membphis]

ok, got it. we can close this issue now.