spec: overwrite original ngx.header in tests that mutate it

The .__newindex method of the ngx.header metatable raises "API disabled
in the current context" when trying to mutate it from the busted tests.
To avoid that, we overwrite ngx.header in the tests that need to modify
it.

spec/policy/cors/cors_spec.lua

@@ -40,8 +40,9 @@ describe('CORS policy', function()
 
   describe('.header_filter', function()
     before_each(function()
-      local headers = {}
-      stub(ngx.header, function() return headers end)
+      -- Replace original ngx.header. Openresty does not allow to modify it when
+      -- running busted tests.
+      ngx.header = {}
     end)
 
     describe('when the policy configuration defines CORS headers', function()

spec/policy/headers/headers_spec.lua

@@ -111,8 +111,9 @@ describe('Headers policy', function()
     local response_headers = 'response' -- header_filter() only modifies resp headers
 
     before_each(function()
-      local headers = {}
-      stub(ngx.header, function() return headers end)
+      -- Replace original ngx.header. Openresty does not allow to modify it when
+      -- running busted tests.
+      ngx.header = {}
     end)
 
     describe('the push operation', function()

spec/proxy_spec.lua

@@ -21,6 +21,10 @@ describe('Proxy', function()
   describe(':rewrite', function()
     local service
     before_each(function()
+      -- Replace original ngx.header. Openresty does not allow to modify it when
+      -- running busted tests.
+      ngx.header = {}
+
       ngx.var = { backend_endpoint = 'http://localhost:1853', uri = '/a/uri' }
       ngx.req = { get_method = function () return 'GET' end}
       service = Service.new({ extract_usage = function() end })