adding test to show that deletes work with mixed valueEncodings
propagating sublevel options properly for deletes
exposing missing support for writeStream mixed valueEncodings

- utf8 and json encoding works fine for per write encodings
- json encoding is broken for per sub encodings
propagating sub level encodings to write streams
using non-checking/propagating write if no encodings were specified on sublevel for optimization
Merge branch 'fix-mixed-valueEncodings-writeStream' of git://github.com/thlorenz/level-sublevel
4.6.13