fix undefined constant GraphQL::Schema::Context

[Stanislav-Lapata]

expanding macro
in macro 'finished' expanded macro: injection:1, line 4:

   1.         #
   2.         # resolve a named field on this object with query arguments and context
   3.         #
>  4.         def resolve_field(name : String, arguments, context : GraphQL::Schema::Context)
   5.           
   6.           
   7.               case name
   8.                   
   9.                     when "__typename" #\\\\\Mafiozki::Api::GraphQL::Fields::User
  10.                       __typename_field(arguments, context)
...
undefined constant GraphQL::Schema::Context