Support rack requests in Slack::Events::Request

The `#headers` method on a request is implemented by Rails, so in
order for the event request class to support other rack based servers
we can instead look for the headers via `#get_header` or `#env`.

Since Rail's requests are an extension of rack requests this will
also work in a Rails app.

Author: wedgex

lib/slack/events/request.rb

@@ -19,13 +19,13 @@ def initialize(http_request, options = {})
 
       # Request timestamp.
       def timestamp
-        @timestamp ||= http_request.headers['X-Slack-Request-Timestamp']
+        @timestamp ||= http_request.get_header('HTTP_X_SLACK_REQUEST_TIMESTAMP')
       end
 
       # The signature is created by combining the signing secret with the body of the request
       # Slack is sending using a standard HMAC-SHA256 keyed hash.
       def signature
-        @signature ||= http_request.headers['X-Slack-Signature']
+        @signature ||= http_request.get_header('HTTP_X_SLACK_SIGNATURE')
       end
 
       # Signature version.

spec/slack/events/request_spec.rb

@@ -21,13 +21,14 @@
     '"P7sFXA4o3HV2hTx4zb4zcQ9yrvuQs8pDh6EacOxmMRj0tJaXfQFF","type":"url_verification"}'
   end
   let(:http_request) do
-    double(
-      headers: {
-        'X-Slack-Request-Timestamp' => timestamp,
-        'X-Slack-Signature' => signature
-      },
+    request_double = double(
       body: StringIO.new(body)
     )
+
+    allow(request_double).to receive(:get_header).with('HTTP_X_SLACK_REQUEST_TIMESTAMP') { timestamp }
+    allow(request_double).to receive(:get_header).with('HTTP_X_SLACK_SIGNATURE') { signature }
+
+    request_double
   end
 
   after do