Fix default javadoc base for jdk11+ (#394)

* Fix default javadoc base for jdk11+

Kind of dodgy that we rely on sys.props.get("java.specification.version")
but ok.

* Specify 'javadoc.java.base_url' explicitly in test

as I think we want to encourage this instead of relying on the
'current java version' anyway

* Default to java.base module for java.*

plugin/src/main/scala/com/lightbend/paradox/sbt/ParadoxPlugin.scala

@@ -277,9 +277,12 @@ object ParadoxPlugin extends AutoPlugin {
     val JavaSpecVersion = """\d+\.(\d+)""".r
     Map(
       "javadoc.java.base_url" -> sys.props.get("java.specification.version").map {
-        case JavaSpecVersion(v) => v
-        case v                  => v
-      }.map { v => url(s"https://docs.oracle.com/javase/$v/docs/api/") },
+        case JavaSpecVersion(v) => v.toInt
+        case v                  => v.toInt
+      }.map { v =>
+        if (v < 11) url(s"https://docs.oracle.com/javase/$v/docs/api/")
+        else url(s"https://docs.oracle.com/en/java/javase/$v/docs/api/java.base/")
+      },
       "scaladoc.version" -> Some(scalaVersion),
       "scaladoc.scala.base_url" -> Some(url(s"http://www.scala-lang.org/api/$scalaVersion")),
       "scaladoc.base_url" -> apiURL,

plugin/src/sbt-test/paradox/parameterized-links/build.sbt

@@ -13,6 +13,7 @@ paradoxProperties in Compile ++= Map(
   "scaladoc.akka.http.base_url" -> s"http://doc.akka.io/api/akka-http/$akkaHttpVersion",
   "javadoc.link_style" -> "frames",
   "javadoc.base_url" -> s"https://api.example.com/java",
+  "javadoc.java.base_url" -> "https://docs.oracle.com/javase/8/docs/api/",
   "javadoc.akka.base_url" -> s"http://doc.akka.io/japi/akka/$akkaVersion",
   "javadoc.akka.http.base_url" -> s"http://doc.akka.io/japi/akka-http/$akkaHttpVersion"
 )
@@ -33,5 +34,5 @@ TaskKey[Unit]("checkJavadocJavalibContent") := {
   assert(file.exists, s"${file.getAbsolutePath} did not exist")
   val content = IO.readLines(file).mkString
   assert(content.matches(
-    raw"""<p><a href="https://docs.oracle.com/javase/\d+/docs/api/\?java/io/File\.html#separator" title="java.io.File"><code>File\.separator</code></a></p>"""))
+    raw"""<p><a href="https://docs.oracle.com/javase/8/docs/api/\?java/io/File\.html#separator" title="java.io.File"><code>File\.separator</code></a></p>"""))
 }