[geary] Add ContentType.is_default and unit test.

[Michael Gratton <mjog src gnome org>]

commit 183da8bd3f5bab055482e2f1d05844252ea1e59f
Author: Michael James Gratton <mike vee net>
Date:   Wed Feb 15 17:50:13 2017 +1100

    Add ContentType.is_default and unit test.

 src/engine/mime/mime-content-type.vala  |    9 ++++++++-
 test/engine/mime-content-type-test.vala |    4 ++++
 2 files changed, 12 insertions(+), 1 deletions(-)
---
diff --git a/src/engine/mime/mime-content-type.vala b/src/engine/mime/mime-content-type.vala
index 6f7350b..59d530b 100644
--- a/src/engine/mime/mime-content-type.vala
+++ b/src/engine/mime/mime-content-type.vala
@@ -233,7 +233,14 @@ public class Geary.Mime.ContentType : Geary.BaseObject {
         
         return is_type(mime_media_type, mime_media_subtype);
     }
-    
+
+    /**
+     * Determines if this type is the same as the default content type.
+     */
+    public bool is_default() {
+        return get_mime_type() == DEFAULT_CONTENT_TYPE;
+    }
+
     public string serialize() {
         StringBuilder builder = new StringBuilder();
         builder.append_printf("%s/%s", media_type, media_subtype);
diff --git a/test/engine/mime-content-type-test.vala b/test/engine/mime-content-type-test.vala
index 6e4c418..951cffa 100644
--- a/test/engine/mime-content-type-test.vala
+++ b/test/engine/mime-content-type-test.vala
@@ -9,10 +9,14 @@ class Geary.Mime.ContentTypeTest : Gee.TestCase {
 
     public ContentTypeTest() {
         base("Geary.Mime.ContentTypeTest");
+        add_test("is_default", is_default);
         add_test("get_file_name_extension", get_file_name_extension);
         add_test("guess_type_from_name", guess_type_from_name);
         add_test("guess_type_from_buf", guess_type_from_buf);
     }
+
+    public void is_default() {
+        assert(new ContentType("application", "octet-stream", null).is_default());
     }
 
     public void get_file_name_extension() {