[issue#336] Fix a generic type mismatch

Author: luan-cestari

src/main/java/com/sendgrid/helpers/mail/Mail.java

@@ -486,7 +486,7 @@ private <K,V> Map<T,V> addToMap(K key, V value, Map<K,V> defaultMap) {
       defaultMap.put(key, value);
       return defaultMap;
     } else {
-      Map<T,V> map = new HashMap<K,V>();
+      Map<K,V> map = new HashMap<K,V>();
       map.put(key, value);
       return map;
     }