[SPARK-7498] [MLLIB] add varargs back to setDefault

We removed `varargs` due to Java compilation issues. That was a false alarm because I didn't run `build/sbt clean`. So this PR reverts the changes. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes apache#6320 from mengxr/SPARK-7498 and squashes the following commits:

74a7259 [Xiangrui Meng] add varargs back to setDefault

mllib/src/main/scala/org/apache/spark/ml/param/params.scala

@@ -438,19 +438,18 @@ trait Params extends Identifiable with Serializable {
    * @param value  the default value
    */
   protected final def setDefault[T](param: Param[T], value: T): this.type = {
-    defaultParamMap.put(param, value)
+    defaultParamMap.put(param -> value)
     this
   }
 
   /**
    * Sets default values for a list of params.
    *
-   * Note: Java developers should use the single-parameter [[setDefault()]].
-   *       Annotating this with varargs causes compilation failures. See SPARK-7498.
    * @param paramPairs  a list of param pairs that specify params and their default values to set
    *                    respectively. Make sure that the params are initialized before this method
    *                    gets called.
    */
+  @varargs
   protected final def setDefault(paramPairs: ParamPair[_]*): this.type = {
     paramPairs.foreach { p =>
       setDefault(p.param.asInstanceOf[Param[Any]], p.value)
@@ -559,7 +558,7 @@ final class ParamMap private[ml] (private val map: mutable.Map[Param[Any], Any])
   /**
    * Puts a (param, value) pair (overwrites if the input param exists).
    */
-  def put[T](param: Param[T], value: T): this.type = put(ParamPair(param, value))
+  def put[T](param: Param[T], value: T): this.type = put(param -> value)
 
   /**
    * Puts a list of param pairs (overwrites if the input params exists).

mllib/src/test/java/org/apache/spark/ml/param/JavaTestParams.java

@@ -81,5 +81,6 @@ private void init() {
       ParamValidators.inArray(validStrings));
     setDefault(myIntParam_, 1);
     setDefault(myDoubleParam_, 0.5);
+    setDefault(myIntParam().w(1), myDoubleParam().w(0.5));
   }
 }