test: add case for overriding existing headers

tests/execution/test-response-header-merge.md

@@ -0,0 +1,31 @@
+# test-response-header-value
+
+```graphql @server
+schema @server(headers: {custom: [{key: "a", value: "a"}]}) {
+  query: Query
+}
+
+type User {
+  name: String
+  age: Int
+}
+
+type Query {
+  user: User @const(data: {name: "John"})
+}
+```
+
+```graphql @server
+schema @server(headers: {custom: [{key: "a", value: "b"}]}) {
+  query: Query
+}
+
+type User {
+  name: String
+  age: Int
+}
+
+type Query {
+  user: User @const(data: {name: "John"})
+}
+```

tests/snapshots/execution_spec__test-response-header-merge.md_merged.snap

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+---
+source: tests/execution_spec.rs
+expression: merged
+---
+schema @server(headers: {custom: [{key: "a", value: "b"}]}) @upstream {
+  query: Query
+}
+
+type Query {
+  user: User @const(data: {name: "John"})
+}
+
+type User {
+  age: Int
+  name: String
+}