error: unmappable character for encoding MS949

When you generate Javadoc with Gradle, you might encounter the following error:

error: unmappable character for encoding MS949

Add the following configuration to your `build.gradle`:

javadoc {
    options.encoding = 'UTF-8'
}

Reference:
http://stackoverflow.com/questions/25912190/how-to-set-an-encoding-for-the-javadoc-in-gradle