Headers key not present in validation

[bbangert]

Somehow the headers key was missing from validation per a traceback on sentry.

KeyError: 'headers'
  File "twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "autopush/web/validation.py", line 59, in _validate_request
    return schema.load(data)
  File "marshmallow/schema.py", line 542, in load
    result, errors = self._do_load(data, many, partial=partial, postprocess=True)
  File "marshmallow/schema.py", line 627, in _do_load
    field_errors=field_errors)
  File "marshmallow/schema.py", line 835, in _invoke_validators
    pass_original=pass_original)
  File "marshmallow/marshalling.py", line 182, in run_validator
    res = validator_func(output)
  File "autopush/web/validation.py", line 270, in ensure_encoding_with_data
    if d["body"] and not all([x in d["headers"] for x in req_fields]):