Add support for custom HTTP predelivery callbacks

This approach will allow us to define last minute callbacks before
performing the HTTP delivery to webhook consumers. Now in the webhook
application that relies on github-services, we can do something like
this:

```ruby
service = Service::Web.new

if webhook.needs_signed_header?
  service.before_delivery do |url, payload, headers, params|
    signature = public_key.sign(message: payload)
    headers['GITHUB-PUBLIC-KEY-SIGNATURE'] = signature
  end
end
```

Author: bswinnerton

lib/service.rb

@@ -485,6 +485,7 @@ def inherited(svc)
   attr_reader :remote_calls
 
   attr_accessor :needs_public_key_signature, :public_key
+  attr_reader :pre_delivery_callbacks
 
   def initialize(event = :push, data = {}, payload = nil)
     helper_name = "#{event.to_s.classify}Helpers"
@@ -502,6 +503,7 @@ def initialize(event = :push, data = {}, payload = nil)
     @http = @secrets = @email_config = nil
     @http_calls = []
     @remote_calls = []
+    @pre_delivery_callbacks = []
   end
 
   # Boolean fields as either nil, "0", or "1".
@@ -601,7 +603,7 @@ def http_get(url = nil, params = nil, headers = nil)
   #
   # Yields a Faraday::Request instance.
   # Returns a Faraday::Response instance.
-  def http_post(url = nil, body = nil, headers = nil, params = nil)
+  def http_post(url = nil, body = nil, headers = {}, params = {})
     block = Proc.new if block_given?
     http_method :post, url, body, headers, params, &block
   end
@@ -633,18 +635,22 @@ def http_post(url = nil, body = nil, headers = nil, params = nil)
   #
   # Yields a Faraday::Request instance.
   # Returns a Faraday::Response instance.
-  def http_method(method, url = nil, body = nil, headers = nil, params = nil)
+  def http_method(method, url = nil, body = nil, headers = {}, params = {})
     block = Proc.new if block_given?
 
     url = url.strip if url
     raise_config_error("Invalid scheme") unless permitted_transport?(url)
 
+    if pre_delivery_callbacks.any?
+      pre_delivery_callbacks.each { |c| c.call(url, body, headers, params) }
+    end
+
     check_ssl do
       http.send(method) do |req|
         req.url(url)                if url
-        req.headers.update(headers) if headers
+        req.headers.update(headers) if headers.present?
         req.body = body             if body
-        req.params = params         if params
+        req.params = params         if params.present?
         block.call req if block
       end
     end
@@ -854,6 +860,10 @@ def reportable_http_env(env, time)
     }
   end
 
+  def before_delivery(&block)
+    @pre_delivery_callbacks << block
+  end
+
   # Raised when an unexpected error occurs during service hook execution.
   class Error < StandardError
     attr_reader :original_exception