Fix: fallback locale should not result in loop [closes #3]

`getFallbackLocale` returns default locale if the given locale isn't the
default one yet.  Otherwise, `null`.

Based on https://docs.oracle.com/javase/8/docs/api/java/util/ResourceBundle.html

README.md

@@ -68,7 +68,7 @@ val email =
 In SBT:
 
 ```
-val i18n = "com.osinka.i18n" %% "scala-i18n" % "1.0.1"
+val i18n = "com.osinka.i18n" %% "scala-i18n" % "1.0.2"
 ```
 
 # Credits

src/main/scala/Messages.scala

@@ -20,7 +20,7 @@ import java.util.{ResourceBundle, Locale}
   *
   * Messages are formatted with `java.text.MessageFormat`.
   */
-object Messages {
+trait Messages {
   val FileName = "messages"
   val FileExt = "txt"
 
@@ -35,19 +35,22 @@ object Messages {
   }
 }
 
+object Messages extends Messages
+
 // @see https://gist.github.com/alaz/1388917
 // @see http://stackoverflow.com/questions/4659929/how-to-use-utf-8-in-resource-properties-with-resourcebundle
 private[i18n] object UTF8BundleControl extends ResourceBundle.Control {
   val Format = "properties.utf8"
-  val FallbackLocale = new Locale("")
 
   override def getFormats(baseName: String): java.util.List[String] = {
     import collection.JavaConverters._
 
     Seq(Format).asJava
   }
 
-  override def getFallbackLocale(baseName: String, locale: Locale) = FallbackLocale
+  override def getFallbackLocale(baseName: String, locale: Locale) =
+    if (locale == Locale.getDefault) null
+    else Locale.getDefault
 
   override def newBundle(baseName: String, locale: Locale, fmt: String, loader: ClassLoader, reload: Boolean): ResourceBundle = {
     import java.util.PropertyResourceBundle

src/test/scala/messagesSpec.scala

@@ -24,5 +24,14 @@ class messagesSpec extends FunSpec with Matchers {
         Messages("nokey")(RU)
       }
     }
+    it("should throw if no resource") {
+      object NoResourceMessage extends Messages {
+        override val FileName = "nonexistant"
+      }
+
+      intercept[java.util.MissingResourceException] {
+        NoResourceMessage("nokey")(EN)
+      }
+    }
   }
 }

version.sbt

@@ -1,2 +1,2 @@
 
-version in ThisBuild := "1.0.2-SNAPSHOT"
+version in ThisBuild := "1.0.2"