Introduce ::getPropertyInternal method to avoid duplication

Document.cpp implements "getter" methods for some of the
Document object properties.
Some of the body of such methods are identical.

Patch introduces a ::getPropertyInternal private method
that gets rid of this duplication.

Namely the following properties' getters are cleaned up:
- "author", "creationDate", "creator", "keywords",
- "modDate", "producer", "subject" and "title"

No behavior change.

Review-Url: https://codereview.chromium.org/2202283002
diff --git a/AUTHORS b/AUTHORS
index 7336dfd..1425768 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -9,6 +9,7 @@
 # See python fnmatch module documentation for more information.
 
 Andrey Khalyavin <halyavin@chromium.org>
+Antonio Gomes <tonikitoo@igalia.com>
 Brett Wilson <brettw@chromium.org>
 Bruce Dawson <brucedawson@chromium.org>
 Chery Cherian <cherycherian@gmail.com>