Ensure that Jbuilder.encode properly forwards arguments to .new

[flavorjones]

The initializer previously accepted an options hash, but now takes kwargs. So the encode method also needs to accept (and forward) kwargs.

Without this change, existing code like this:

Jbuilder.encode(ignore_nil: true) do |json|
  # ...
end

raises an exception. The test I've added here fails with:

Error:
JbuilderTest#test_encode_forwards_options_to_new:
ArgumentError: wrong number of arguments (given 1, expected 0)
    lib/jbuilder.rb:16:in 'initialize'
    lib/jbuilder.rb:32:in 'Class#new'
    lib/jbuilder.rb:32:in 'Jbuilder.encode'
    test/jbuilder_test.rb:941:in 'block in <class:JbuilderTest>'

Fixes #602