add "http://" scheme to ENV['http_proxy'] value if missing

This allows:

    http_proxy=localhost:8888 ruby myscript.rb

Omitting the scheme is already supported by curl, and this matches that
behavior.

lib/faraday/connection.rb

@@ -80,7 +80,13 @@ module Faraday
       @headers.update(options.headers) if options.headers
 
       @proxy = nil
-      proxy(options.fetch(:proxy) { ENV['http_proxy'] })
+      proxy(options.fetch(:proxy) {
+        uri = ENV['http_proxy']
+        if uri && !uri.empty?
+          uri = 'http://' + uri if uri !~ /^http/i
+          uri
+        end
+      })
 
       yield self if block_given?
 

test/connection_test.rb

@@ -287,6 +287,42 @@ class TestConnection < Faraday::TestCase
     end
   end
 
+  def test_proxy_accepts_env_without_scheme
+    with_env 'http_proxy', "localhost:8888" do
+      uri = Faraday::Connection.new.proxy[:uri]
+      assert_equal 'localhost', uri.host
+      assert_equal 8888, uri.port
+    end
+  end
+
+  def test_no_proxy_from_env
+    with_env 'http_proxy', nil do
+      conn = Faraday::Connection.new
+      assert_equal nil, conn.proxy
+    end
+  end
+
+  def test_no_proxy_from_blank_env
+    with_env 'http_proxy', '' do
+      conn = Faraday::Connection.new
+      assert_equal nil, conn.proxy
+    end
+  end
+
+  def test_proxy_doesnt_accept_uppercase_env
+    with_env 'HTTP_PROXY', "http://localhost:8888/" do
+      conn = Faraday::Connection.new
+      assert_nil conn.proxy
+    end
+  end
+
+  def test_proxy_requires_uri
+    conn = Faraday::Connection.new
+    assert_raises ArgumentError do
+      conn.proxy :uri => :bad_uri, :user => 'rick'
+    end
+  end
+
   def test_dups_connection_object
     conn = Faraday::Connection.new 'http://sushi.com/foo',
       :ssl => { :verify => :none },