use net/http by default if no middleware is set

lib/faraday.rb

@@ -1,6 +1,12 @@
 require 'rack/utils'
 
 module Faraday
+  class << self
+    attr_accessor :default_adapter
+  end
+
+  self.default_adapter = :net_http
+
   module AutoloadHelper
     def register_lookup_modules(mods)
       (@lookup_module_index ||= {}).update(mods)

lib/faraday/builder.rb

@@ -24,7 +24,10 @@ module Faraday
       build(&block) if block_given?
     end
 
-    def build(&block)
+    def build(options = {}, &block)
+      if options[:reset]
+        @handlers.clear
+      end
       block.call(self)
       run(self.class.inner_app)
     end
@@ -39,6 +42,10 @@ module Faraday
     end
 
     def to_app
+      if @handlers.empty?
+        build { |b| b.adapter Faraday.default_adapter }
+      end
+
       inner_app = @handlers.first
       @handlers[1..-1].inject(inner_app) { |app, middleware| middleware.call(app) }
     end

lib/faraday/connection.rb

@@ -44,8 +44,8 @@ module Faraday
       end
     end
 
-    def build(&block)
-      @builder.build(&block)
+    def build(options = {}, &block)
+      @builder.build(options, &block)
     end
 
     def get(url = nil, headers = nil, &block)

test/connection_test.rb

@@ -218,4 +218,13 @@ class TestConnection < Faraday::TestCase
       assert_not_equal conn.send(attr).object_id, duped.send(attr).object_id
     end
   end
-end
+
+  def test_sets_default_adapter_if_none_set
+    conn  = Faraday::Connection.new
+    assert_equal 0, conn.builder.handlers.size
+
+    app   = conn.to_app
+    mware = conn.builder.handlers[1].call({})
+    assert_kind_of Faraday::Adapter::NetHttp, mware
+  end
+end