Fix class_eval without AS

Fixes #9

lib/graphql/client/http.rb

@@ -29,7 +29,7 @@ module GraphQL
       # block - Optional block to configure class
       def initialize(uri, &block)
         @uri = URI.parse(uri)
-        class_eval(&block) if block_given?
+        singleton_class.class_eval(&block) if block_given?
       end
 
       # Public: Parsed endpoint URI

test/test_http.rb

@@ -3,8 +3,10 @@ require "graphql/client/http"
 require "minitest/autorun"
 
 class TestHTTP < MiniTest::Test
-  def setup
-    @http = GraphQL::Client::HTTP.new("http://graphql-swapi.parseapp.com/")
+  SWAPI = GraphQL::Client::HTTP.new("http://graphql-swapi.parseapp.com/") do
+    def headers(_context)
+      { "User-Agent" => "GraphQL/1.0" }
+    end
   end
 
   def test_execute
@@ -28,7 +30,7 @@ class TestHTTP < MiniTest::Test
         }
       }
     }
-    actual = @http.execute(document: document, operation_name: name, variables: variables)
+    actual = SWAPI.execute(document: document, operation_name: name, variables: variables)
     assert_equal(expected, actual)
   end
 end