buffer: fix and cleanup fill()

Running fill() with an empty string would cause Node to hang
indefinitely. Now it will return without having operated on the buffer.

User facing function has been pulled into JS to perform all initial
value checks and coercions. The C++ method has been placed on the
"internal" object.

Coerced non-string values to numbers to match v0.10 support.

Simplified logic and changed a couple variable names.

Added tests for fill() and moved them all to the beginning of
buffer-test.js since many other tests depend on fill() working properly.

Fixes: https://github.com/joyent/node/issues/8469
Signed-off-by: Trevor Norris <trev.norris@gmail.com>