fix PostTest tests
test reprocucing #1544
new testGetNullCustomField
fix #1544: null check customField
Super is now called at the end of onSaveInstanceState() in all reader fragments/activities that use it.
Merge pull request #1554 from wordpress-mobile/issue/1546-reader-list-illegal-state-exception

issue/1546-reader-list-illegal-state-exception
Merge pull request #1548 from wordpress-mobile/issue/1544-npe-getcustomfields

fix #1544 npe in Post.getCustomFields
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into issue/1491-reader-block-blog