(core) Ignore leading whitespace in formulas, and strip out leading '=' sign users might add

Summary:
This addresses two issues, differently:
- For a formula with leading whitespace, like " 1+1", it is stored as is, but
  is fixed to work (it should be valid Python, and whitespace is only stripped out
  at parsing time to avoid intentation errors caused by the way it gets parsed)
- For a formula with a leading equals-sign ("="), it is stripped out on the
  client side before the formula is stored. Grist documentation uses leading
  "=" to indicate formulas (because UI shows an "=" icon), and Excel formulas
  actually contain the leading "=", so it is a common mistake to include it.

Test Plan: Added new test cases

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3873

sandbox/grist/test_codebuilder.py

@@ -1,15 +1,14 @@
 # -*- coding: utf-8 -*-
-import unittest
-
 import codebuilder
 import six
+import test_engine
 
 unicode_prefix = 'u' if six.PY2 else ''
 
 def make_body(formula, default=None):
   return codebuilder.make_formula_body(formula, default).get_text()
 
-class TestCodeBuilder(unittest.TestCase):
+class TestCodeBuilder(test_engine.EngineTestCase):
   def test_make_formula_body(self):
     # Test simple usage.
     self.assertEqual(make_body(""), "return None")
@@ -239,3 +238,19 @@ return x or y
 
     # Check that missing arguments is OK
     self.assertEqual(make_body("ISERR()"), "return ISERR()")
+
+
+  def test_leading_whitespace(self):
+    self.assertEqual(make_body(" $A + 1"), "return rec.A + 1")
+
+    self.assertEqual(make_body("""
+  if $A:
+    return $A
+
+  $B
+"""), """
+if rec.A:
+  return rec.A
+
+return rec.B
+""")