Fix more instance variable not initialized warnings

Since EntryPoint does not call Link#initialize, we must make sure to
initialize all of Link's instance variables to avoid any warnings.

Author: ivoanjo

lib/hyperclient/entry_point.rb

@@ -41,6 +41,8 @@ def initialize(url, &_block)
       @options = { async: true }
       @connection = nil
       @resource = nil
+      @key = nil
+      @uri_variables = nil
       yield self if block_given?
     end