KotlinGenerateToStringAction: should generate .contentToString instead java.util.Arrays.toString

#KT-27563 Fixed

idea/testData/codeInsight/generate/toString/singleTemplate/nullableArrays.kt.after

@@ -0,0 +1,12 @@
+// GENERATOR: SINGLE_TEMPLATE
+class A(val n: IntArray?, val s: Array<String>) {
+    val f: Float = 1.0f
+
+    fun foo() {
+
+    }
+
+    override fun toString(): String {
+        return "A(n=${n?.contentToString()}, s=${s.contentToString()}, f=$f)"
+    }
+}