Fixed load/loadSync filename parameter type
Fixed load/loadSync filename parameter type

Fixes #805.

Changed the type of the filename parameter to the proto-loader load
method to match the type of the protobuf.js load method that it is
passed to.

Signed-off-by: James Roper <474ba67bdb289c6263b36dfd8a7bed6c85b04943@jazzy.id.au>

grpc/grpc-node,grpc/grpc-node,grpc/grpc-node,grpc/grpc-node