[JENKINS-21486] Fixing optional dependency version resolution. (#2421)

Without it version of optional dependencies is set to "X.Y;resolution:=optional" which causes problems when using hudson.util.VersionNumber comparison methods.

core/src/test/java/hudson/PluginWrapperTest.java

@@ -0,0 +1,26 @@
+package hudson;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class PluginWrapperTest {
+
+    @Test
+    public void dependencyTest() {
+        String version = "plugin:0.0.2";
+        PluginWrapper.Dependency dependency = new PluginWrapper.Dependency(version);
+        assertEquals("plugin", dependency.shortName);
+        assertEquals("0.0.2", dependency.version);
+        assertEquals(false, dependency.optional);
+    }
+
+    @Test
+    public void optionalDependencyTest() {
+        String version = "plugin:0.0.2;resolution:=optional";
+        PluginWrapper.Dependency dependency = new PluginWrapper.Dependency(version);
+        assertEquals("plugin", dependency.shortName);
+        assertEquals("0.0.2", dependency.version);
+        assertEquals(true, dependency.optional);
+    }
+}