db_api: parse_insert method doesn't work as expected

[HemangChothani]

As mentioned in the docstring Case c return the dict with the info like homogenous, homogenous etc:

python-spanner/google/cloud/spanner_dbapi/parse_utils.py

Lines 235 to 244 in 1f80a39

	Case c)
	SQL: INSERT INTO T (f1, f2) VALUES (%s, %s), (%s, %s)
	Params: ['a', 'b', 'c', 'd']
	it produces:
	{
	'homogenous': True,
	'table': 'T',
	'columns': ['f1', 'f2'],
	'values': [('a', 'b',), ('c', 'd',)],
	}

Where code of Case c returns sql_params_list:

python-spanner/google/cloud/spanner_dbapi/parse_utils.py

Lines 305 to 321 in 1f80a39

	if values.homogenous():
	# Case c)
	columns = [mi.strip(" `") for mi in match.group("columns").split(",")]
	sql_params_list = []
	insert_sql_preamble = "INSERT INTO %s (%s) VALUES %s" % (
	match.group("table_name"),
	match.group("columns"),
	values.argv[0],
	)
	values_pyformat = [str(arg) for arg in values.argv]
	rows_list = rows_for_insert_or_update(columns, params, values_pyformat)
	insert_sql_preamble = sanitize_literals_for_upload(insert_sql_preamble)
	for row in rows_list:
	sql_params_list.append((insert_sql_preamble, row))
	return {"sql_params_list": sql_params_list}

So it never execute for the homogeneous insert

python-spanner/google/cloud/spanner_dbapi/connection.py

Line 306 in 1f80a39

_execute_insert_homogenous(transaction, parts)

cc:// @larkee @c24t