SendGrid: convert long to str in headers, metadata

SendGrid requires extra headers and metadata values be strings.
Anymail has always coerced int and float; this treats Python 2's
`long` integer type the same.

Fixes #74

anymail/utils.py

@@ -18,6 +18,10 @@ from six.moves.urllib.parse import urlsplit, urlunsplit
 
 from .exceptions import AnymailConfigurationError, AnymailInvalidAddress
 
+
+BASIC_NUMERIC_TYPES = six.integer_types + (float,)  # int, float, and (on Python 2) long
+
+
 UNSET = object()  # Used as non-None default value