Fix webhook request for headers with symbol key

E.g. the clearance session is called `:clearance` and it then fails with the error:

```
ShopifyAPITest::Webhooks::WebhookRequestTest#test_with_symbol_headers:
NoMethodError: undefined method `sub' for :clearance:Symbol
Did you mean?  stub
```

Author: phylor

lib/shopify_api/webhooks/request.rb

@@ -35,7 +35,7 @@ def parsed_body
       sig { params(raw_body: String, headers: T::Hash[String, T.untyped]).void }
       def initialize(raw_body:, headers:)
         # normalize the headers by forcing lowercase, removing any prepended "http"s, and changing underscores to dashes
-        headers = headers.to_h { |k, v| [k.downcase.sub("http_", "").gsub("_", "-"), v] }
+        headers = headers.to_h { |k, v| [k.to_s.downcase.sub("http_", "").gsub("_", "-"), v] }
 
         missing_headers = []
         [

test/webhooks/request_test.rb

@@ -21,6 +21,17 @@ def test_error_when_headers_missing
           ShopifyAPI::Webhooks::Request.new(raw_body: "{}", headers: {})
         end
       end
+
+      def test_with_symbol_headers
+        headers = {
+          "HTTP_X_SHOPIFY_TOPIC" => "some/topic",
+          "HTTP_X_SHOPIFY_HMAC_SHA256" => "some_hmac",
+          "HTTP_X_SHOPIFY_SHOP_DOMAIN" => "shop.myshopify.com",
+          :clearance => "session"
+        }
+
+        assert(ShopifyAPI::Webhooks::Request.new(raw_body: "{}", headers: headers))
+      end
     end
   end
 end