Allow pure Ruby and Java NIO::Selector to be initialized with nil

Trying to make this consistent across all backends. Passing nil was only
working with libev.

ext/nio4r/org/nio4r/Selector.java

@@ -43,7 +43,7 @@ public class Selector extends RubyObject {
 
     @JRubyMethod
     public IRubyObject initialize(ThreadContext context, IRubyObject backend) {
-        if(backend != context.runtime.newSymbol("java")) {
+        if(backend != context.runtime.newSymbol("java") && !backend.isNil()) {
             throw context.runtime.newArgumentError(":java is the only supported backend");
         }
 

lib/nio/selector.rb

@@ -14,7 +14,7 @@ module NIO
 
     # Create a new NIO::Selector
     def initialize(backend = :ruby)
-      raise ArgumentError, "unsupported backend: #{backend}" unless backend == :ruby
+      raise ArgumentError, "unsupported backend: #{backend}" unless [:ruby, nil].include?(backend)
 
       @selectables = {}
       @lock = Mutex.new

spec/nio/selector_spec.rb

@@ -24,6 +24,10 @@ RSpec.describe NIO::Selector do
       example.reporter.message "Supported backends: #{described_class.backends}"
     end
 
+    it "automatically selects a backend if none or nil is specified" do
+      expect(described_class.new.backend).to eq described_class.new(nil).backend
+    end
+
     it "raises ArgumentError if given an invalid backend" do
       expect { described_class.new(:derp) }.to raise_error ArgumentError
     end

spec/spec_helper.rb

@@ -12,6 +12,8 @@ RSpec.configure do |config|
   # Enable flags like --only-failures and --next-failure
   config.example_status_persistence_file_path = ".rspec_status"
 
+  config.filter_run_when_matching :focus
+
   config.expect_with :rspec do |c|
     c.syntax = :expect
   end