failing tests for issues related to https://github.com/tessel/runtime/issues/690
properly pass along encoding on buf.write, fixes #690
somewhat improved length handling, a few more tests pass
disable unrelated failing test (tracking as https://github.com/tessel/runtime/issues/691)
half fix for UTF-16 length handling
make UTF-16 truncation workaround a little more reliable
update count after disabled test
avoid leftover tonumber (inconsistent handling versus other params)
Merge #692 r=@johnnyman727: Buffer.prototype.write encoding fixes

Fixes #690 and some other write method parameter handling.