Refactored to add a way to copy JobParameters into another via builder

In a previous commit, the JobParametersBuilder was updated to include
some code from Spring Boot that handled the incrementing of
JobParameters for a previous job.  That commit brought over a private
`merge` method that is actually useful for general consumption.

This commit adds a `addJobParameters` method to the builder providing
the same functionality the `merge` method did in a public method.

spring-batch-core/src/test/java/org/springframework/batch/core/JobParametersBuilderTests.java

@@ -64,6 +64,28 @@ public class JobParametersBuilderTests {
 		this.parametersBuilder = new JobParametersBuilder(this.jobExplorer);
 	}
 
+	@Test
+	public void testAddingExistingJobParameters() {
+		JobParameters params1 = new JobParametersBuilder()
+				.addString("foo", "bar")
+				.addString("bar", "baz")
+				.toJobParameters();
+
+		JobParameters params2 = new JobParametersBuilder()
+				.addString("foo", "baz")
+				.toJobParameters();
+
+		JobParameters finalParams = new JobParametersBuilder()
+				.addString("baz", "quix")
+				.addJobParameters(params1)
+				.addJobParameters(params2)
+				.toJobParameters();
+
+		assertEquals(finalParams.getString("foo"), "baz");
+		assertEquals(finalParams.getString("bar"), "baz");
+		assertEquals(finalParams.getString("baz"), "quix");
+	}
+
 	@Test
 	public void testNonIdentifyingParameters() {
 		this.parametersBuilder.addDate("SCHEDULE_DATE", date, false);