Prefer Headers#add over Headers#[]= for adding headers.

Author: samuel-williams-shopify

lib/protocol/rack/adapter/rack2.rb

@@ -131,10 +131,10 @@ def wrap_headers(fields)
 							meta[key] = value
 						elsif value.is_a?(String)
 							value.split("\n").each do |value|
-								headers[key] = value
+								headers.add(key, value)
 							end
 						else
-							headers[key] = value
+							headers.add(key, value)
 						end
 					end
 

lib/protocol/rack/adapter/rack3.rb

@@ -103,10 +103,10 @@ def wrap_headers(fields)
 							meta[key] = value
 						elsif value.is_a?(Array)
 							value.each do |value|
-								headers[key] = value
+								headers.add(key, value)
 							end
 						else
-							headers[key] = value
+							headers.add(key, value)
 						end
 					end
 

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - Use `Headers#add` instead of `Headers#[]=` in Rack3 and Rack31 adapters, which is the correct interface for appending headers.
+
 ## v0.18.0
 
   - Correctly invoke `rack.response_finished` in reverse order.