Fix bug #1163572

RTF input modifications
RTF Input: Use input encoding setting for files with no codepage

RTF Input: When converting RTF files with no codepage, use the input
encoding setting as the codepage. Fixes #1163572 [RTF charset ansicpg0 handling](https://bugs.launchpad.net/calibre/+bug/1163572)

Merge branch 'RTF-changes' of https://github.com/sengian/calibre