Extract utility methods to the StringUtil class

and ignore case when comparing Accept/Content-Type

modules/swagger-codegen/src/main/resources/Java/StringUtil.mustache

@@ -0,0 +1,23 @@
+package {{invokerPackage}};
+
+public class StringUtil {
+  public static boolean containsIgnoreCase(String[] array, String value) {
+    for (String str : array) {
+      if (value == null && str == null) return true;
+      if (value != null && value.equalsIgnoreCase(str)) return true;
+    }
+    return false;
+  }
+
+  public static String join(String[] array, String separator) {
+    int len = array.length;
+    if (len == 0) return "";
+
+    StringBuilder out = new StringBuilder();
+    out.append(array[0]);
+    for (int i = 1; i < len; i++) {
+      out.append(separator).append(array[i]);
+    }
+    return out.toString();
+  }
+}