Change Builder from a class to a module and only build app once

[speckins]

Previously, OmniAuth::Builder was a subclass of Rack::Builder. Rack::Builder#call calls #to_app, rebuilding the Rack application on each request, and, as it says in the Rack::Builder comments, this may have a negative impact on performance.

This commit changes OmniAuth::Builder from a class to a module and defines a ::new method that extends a Rack::Builder instance with that module, instance_evals any block, and returns the value of #to_app, the built application.

Using OmniAuth::Builder will now only build the application once.

For users who have been subclassing OmniAuth::Builder, a fix would be subclassing Rack::Builder directly and including OmniAuth::Builder:

class MyBuilder < Rack::Builder
  include OmniAuth::Builder
  # ...
end

Resolves #1083.

[BobbyMcWho]

Do we need to pass an app in here? Can we default it to nil like Rack::Builder does in internal code?

[speckins]

Technically, no, but if no @run (in Rack::Builder internals) is defined by the time #to_app is called, an error will be raised, and the only way to add one would be to call #run in the block.

OmniAuth::Builder.new(nil) do
  provider :developer
  run proc {}
end

If you needed that, maybe we could extract the first two lines to a separate module method (e.g., ::build) that didn't require an argument.

def self.build(app = nil, &block)
  builder = Rack::Builder.new(app).extend(self)
  builder.instance_eval(&block) if block_given?
  builder
end

def self.new(app, &block)
  build(app, &block).to_app
end

[BobbyMcWho]

Suggested change

	def self.new app, &block
	def self.new(app, &block)