case insensitive header check

setup.py

@@ -2,7 +2,7 @@
 
 setuptools.setup(
     name="simplegmail",
-    version="4.0.2",
+    version="4.0.3",
     url="https://github.com/jeremyephron/simplegmail",
     author="Jeremy Ephron",
     author_email="[email protected]",

simplegmail/gmail.py

@@ -693,16 +693,16 @@ def _build_message_from_ref(
             subject = ''
             msg_hdrs = {}
             for hdr in headers:
-                if hdr['name'] == 'Date':
+                if hdr['name'].lower() == 'date':
                     try:
                         date = str(parser.parse(hdr['value']).astimezone())
                     except Exception:
                         date = hdr['value']
-                elif hdr['name'] == 'From':
+                elif hdr['name'].lower() == 'from':
                     sender = hdr['value']
-                elif hdr['name'] == 'To':
+                elif hdr['name'].lower() == 'to':
                     recipient = hdr['value']
-                elif hdr['name'] == 'Subject':
+                elif hdr['name'].lower() == 'subject':
                     subject = hdr['value']
 
                 msg_hdrs[hdr['name']] = hdr['value']