add helper to determine content type

Author: matthewshirley

lib/src/utils/content_type.dart

@@ -0,0 +1,8 @@
+String getContentType<T>(content) {
+  switch (T) {
+    case Map<String, dynamic>:
+      return 'application/json';
+    default:
+      return 'text/plain';
+  }
+}