fix(fetch/axios): correctly merge headers from opts (#231)

- Previously if you passed any request headers using the `opts`
parameter, this would overwrite the headers set from parameters,
defaults, etc
- Now these will be consistently merged following: `defaults` ->
`params` -> `opts`
- `undefined` header values will be ignored, `null` header values will
delete headers set previously

This wound up being a lot more complex than I'd expected, and there may
still be some edges that I've missed. However I'm confident that this
makes things better than before based on the new e2e tests at least.

e2e/openapi.yaml

@@ -3,8 +3,32 @@ info:
   title: E2E Tests for @nahkies/openapi-code-generator
   version: "0.0.1"
 tags:
+  - name: headers
   - name: validation
 paths:
+  /headers/undeclared:
+    get:
+      tags:
+        - headers
+      responses:
+        200:
+          $ref: '#/components/responses/GetHeaders'
+  /headers/request:
+    get:
+      tags:
+        - headers
+      parameters:
+        - name: route-level-header
+          in: header
+          schema:
+            type: string
+        - name: Authorization
+          in: header
+          schema:
+            type: string
+      responses:
+        200:
+          $ref: '#/components/responses/GetHeaders'
   /validation/numbers/random-number:
     get:
       tags:
@@ -38,6 +62,22 @@ paths:
                 $ref: '#/components/schemas/RandomNumber'
 
 components:
+  responses:
+    GetHeaders:
+      description: success
+      content:
+        application/json:
+          schema:
+            type: object
+            properties:
+              headers:
+                additionalProperties:
+                  anyOf:
+                    - type: string
+                    - type: array
+                      items:
+                        type: string
+
   schemas:
     RandomNumber:
       type: object

e2e/scripts/generate.sh

@@ -12,8 +12,14 @@ yarn openapi-code-generator \
 
 yarn openapi-code-generator \
   --input ./openapi.yaml \
-  --output ./src/generated/client \
+  --output ./src/generated/client/fetch \
   --template typescript-fetch \
   --schema-builder zod \
-  --extract-inline-schemas \
-  --grouping-strategy first-tag \
+  --extract-inline-schemas
+
+yarn openapi-code-generator \
+  --input ./openapi.yaml \
+  --output ./src/generated/client/axios \
+  --template typescript-axios \
+  --schema-builder zod \
+  --extract-inline-schemas