Add default constructor to CaseInsensitiveMap so JRuby can dup() it.

I ran into this on 1.5.0.preview.7 when trying to pass an attributes map obtained from `DocumentHeader.getAttributes()` into `readDocumentStructure()`:
````java
import org.asciidoctor.Asciidoctor;
import org.asciidoctor.OptionsBuilder;
import org.asciidoctor.ast.DocumentHeader;

public class Test {

    public static void main(String[] args) {
        Asciidoctor asciidoctor = Asciidoctor.Factory.create();
        DocumentHeader hdr = asciidoctor.readDocumentHeader("foo");
        asciidoctor.readDocumentStructure("bar", OptionsBuilder.options().attributes(hdr.getAttributes()).asMap());
    }
}
````

JRuby blows up with this error:
````
Exception in thread "main" org.jruby.exceptions.RaiseException: (NotImplementedError) can't dup Map of type org.asciidoctor.internal.CaseInsensitiveMap
	at org.jruby.RubyKernel.dup(org/jruby/RubyKernel.java:2062)
	at RUBY.load(jar:file:/home/ben/.gradle/caches/modules-2/files-2.1/org.asciidoctor/asciidoctorj/1.5.0.preview.7/946801c962a23808795becce2f6a27f44674230c/asciidoctorj-1.5.0.preview.7.jar!/gems/asciidoctor-1.5.0.preview.7/lib/asciidoctor.rb:1217)
	at RUBY.load(<script>:74)
	at org.jruby.gen.InterfaceImpl367913758.load(org/jruby/gen/InterfaceImpl367913758.gen:13)
````

Found the problem in JRuby's [MapJavaProxy](https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/java/proxies/MapJavaProxy.java#L615), it wants to call the Map classes' default constructor via reflection, but `CaseInsensitiveMap` does not have a default constructor, so it fails.

Adding the default constructor fixes this issue.