Do not fold imports if there is only one

#KT-3410 Fixed

idea/src/org/jetbrains/jet/plugin/JetFoldingBuilder.java

@@ -45,7 +45,7 @@ public class JetFoldingBuilder extends FoldingBuilderEx implements DumbAware {
         JetFile file = (JetFile) root;
 
         List<JetImportDirective> importList = file.getImportDirectives();
-        if (importList != null && !importList.isEmpty()) {
+        if (importList != null && importList.size() > 1) {
             JetImportDirective firstImport = importList.get(0);
             PsiElement importKeyword = firstImport.getFirstChild();
             int startOffset = importKeyword.getTextRange().getEndOffset() + 1;

idea/testData/folding/oneImport.kt

@@ -0,0 +1,3 @@
+import kotlin.*
+
+fun foo() {}

idea/tests/org/jetbrains/jet/plugin/folding/KotlinFoldingTestGenerated.java

@@ -56,4 +56,9 @@ public class KotlinFoldingTestGenerated extends AbstractKotlinFoldingTest {
         doTest("idea/testData/folding/object.kt");
     }
     
+    @TestMetadata("oneImport.kt")
+    public void testOneImport() throws Exception {
+        doTest("idea/testData/folding/oneImport.kt");
+    }
+    
 }