fix: email digest original_user issue

[pateljannat]

Steps to replicate:

1. Create an email digest. Save.
2. From the top of the document click on Send Now.
3. An error appears.

Traceback

Traceback (most recent call last):
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/app.py", line 68, in application
    response = frappe.api.handle()
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/api.py", line 55, in handle
    return frappe.handler.handle()
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/handler.py", line 31, in handle
    data = execute_cmd(cmd)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/handler.py", line 67, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/__init__.py", line 1172, in call
    return fn(*args, **newargs)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/handler.py", line 234, in run_doc_method
    response = doc.run_method(method)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/model/document.py", line 861, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/model/document.py", line 1153, in composer
    return composed(self, method, *args, **kwargs)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/model/document.py", line 1136, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/frappe/frappe/model/document.py", line 855, in 
    fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
  File "/home/frappe/benches/bench-version-13-2021-08-19/apps/erpnext/erpnext/setup/doctype/email_digest/email_digest.py", line 63, in send
    frappe.set_user(original_user)
NameError: name 'original_user' is not defined
Request Data
{
	"type": "POST",
	"args": {
  "docs": "{\"name\":\"Default Weekly Digest - Technology ERA Company\",\"owner\":\"yalturkmani@teraksa.com\",\"creation\":\"2018-11-26 17:22:26.206951\",\"modified\":\"2020-09-03 00:15:05.078892\",\"modified_by\":\"yalturkmani@teraksa.com\",\"idx\":0,\"docstatus\":0,\"enabled\":1,\"company\":\"Technology ERA Company\",\"frequency\":\"Daily\",\"next_send\":\"27-08-2021 at midnight\",\"income\":1,\"expenses_booked\":1,\"income_year_to_date\":1,\"expense_year_to_date\":1,\"bank_balance\":1,\"credit_balance\":1,\"invoiced_amount\":1,\"payables\":1,\"sales_orders_to_bill\":0,\"purchase_orders_to_bill\":0,\"sales_order\":1,\"purchase_order\":1,\"sales_orders_to_deliver\":0,\"purchase_orders_to_receive\":0,\"sales_invoice\":1,\"purchase_invoice\":1,\"new_quotations\":1,\"pending_quotations\":1,\"issue\":1,\"project\":1,\"purchase_orders_items_overdue\":0,\"calendar_events\":0,\"todo_list\":0,\"notifications\":0,\"add_quote\":0,\"doctype\":\"Email Digest\",\"recipients\":[{\"name\":\"438678712e\",\"owner\":\"Administrator\",\"creation\":\"2021-08-20 20:08:15.084572\",\"modified\":\"2021-08-20 20:08:15.084572\",\"modified_by\":\"Administrator\",\"parent\":\"Default Weekly Digest - Technology ERA Company\",\"parentfield\":\"recipients\",\"parenttype\":\"Email Digest\",\"idx\":1,\"docstatus\":0,\"recipient\":\"yalturkmani@teraksa.com\",\"doctype\":\"Email Digest Recipient\"}],\"__last_sync_on\":\"2021-08-25T21:02:59.418Z\"}",
  "method": "send",
  "args": null
	},
	"headers": {},
	"error_handlers": {},
	"url": "/api/method/run_doc_method"
}

[coveralls]

Coverage remained the same at 51.596% when pulling 3acc5c6 on pateljannat:email-digest-issue into 71b7c63 on frappe:develop.