fix: Fix check so it's all lowercase. (#321)

Fixes #319 

- [x] Tests pass
- [x] Appropriate changes to README are included in PR

Author: jakehilton

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -182,6 +182,7 @@ class SpannerIdentifierPreparer(IdentifierPreparer):
 
     reserved_words = RESERVED_WORDS.copy()
     reserved_words.update(spanner_dbapi.parse_utils.SPANNER_RESERVED_KEYWORDS)
+    reserved_words_lc = set(map(str.lower, reserved_words))
 
     def __init__(self, dialect):
         super(SpannerIdentifierPreparer, self).__init__(
@@ -195,7 +196,7 @@ def _requires_quotes(self, value):
             return False
 
         return (
-            lc_value in self.reserved_words
+            lc_value in self.reserved_words_lc
             or value[0] in self.illegal_initial_characters
             or not self.legal_characters.match(str(value))
             or (lc_value != value)