RFR: JDK-8188649: javadoc -encoding doesn't work when using the old doclet API

[Jonathan Gibbons]

Please review a trivial change to fix the handling of the -encoding 
option when an old-style doclet is used.

A test for the fixed behavior is added, along with an additional 
equivalent test for when a new-style doclet is used.

JBS: https://bugs.openjdk.java.net/browse/JDK-8188649
Webrev: http://cr.openjdk.java.net/~jjg/8188649/webrev.00

-- Jon