Path segments in base_url can get clobbered by UrlHelpers

[alassek]

Expected Result

Configuring a base_url with a path prefix should preserve it when generating URLs

base_url = URI("http://example.com/hanami")
router = Hanami::Router.new(base_url: base_url) do
  get "/test", to: ->(env) { [200, {}, ["OK"]] }, as: :test
end
router.url(:test) # => #<URI::HTTP http://example.com/hanami/test>

Actual Result

When base_url is a URI object, the path prefix gets replaced by the route path.

base_url = URI("http://example.com/hanami")
router = Hanami::Router.new(base_url: base_url) do
  get "/test", to: ->(env) { [200, {}, ["OK"]] }, as: :test
end
router.url(:test) # => #<URI::HTTP http://example.com/test>

Further Context

There is a type mismatch in how base_url is being configured. In this project, it is being tested with a string

router/spec/unit/hanami/router/url_spec.rb

Lines 4 to 16 in a4f0242

	let(:router) do
	e = endpoint
	Hanami::Router.new(base_url: base_url) do
	get "/hanami", to: e, as: :fixed
	get "/flowers/:id", to: e, as: :variables
	get "/books/:id", id: /\d+/, to: e, as: :constraints
	get "/articles(.:format)", to: e, as: :optional
	get "/files/*glob", to: e, as: :glob
	end
	end
	let(:endpoint) { ->(*) { [200, {}, ["Hi!"]] } }
	let(:base_url) { "https://hanami.test" }

However, in the framework proper the base_url configuration is being coerced to a URI

https://github.com/hanami/hanami/blob/6780d4ea87b54ce543c8aa18fed0e46b8fbf84d6/lib/hanami/config.rb#L115-L128

The UrlHelpers class constructs a URL combining base_url with the generated path using +

router/lib/hanami/router/url_helpers.rb

Lines 34 to 38 in a4f0242

	# @since 2.0.0
	# @api private
	def url(name, variables = {})
	@base_url + path(name, variables)
	end

When base_url is a string, this is a simple string concatenation. When base_url is a URI, + is an alias for merge which interprets a path beginning with / as absolute.