title: ReloadableResourceBundleMessageSource unable to find message when default locale is "en"
body: <p>Why is the spring "ReloadableResourceBundleMessageSource" unable to find the proper message associated with a code when Locale.getDefault() returns <strong>en</strong>, but able to find the proper message when it returns <strong>en_US</strong></p>
  
  <p><strong>Default Locale: en_US</strong></p>
  
  <pre><code>key: CODE_1, Locale: Locale.US           = Hit
  key: CODE_1, Locale: Locale.CANADA       = Hit
  key: CODE_1, Locale: fr_CA               = Hit
  key: CODE_1, Locale: null (use default)  = Hit
  </code></pre>
  
  <p><strong>Default Locale: en</strong></p>
  
  <pre><code>key: CODE_1, Locale: Locale.US               = Hit
  key: CODE_1, Locale: Locale.CANADA           = CODE_1
  key: CODE_1, Locale: fr_CA                   = CODE_1
  key: CODE_1, Locale: null (use default)      = CODE_1
  </code></pre>
  
  <p>I have only one bundle that has the CODE_1 in the classpath which is <strong>message_en_US.properties</strong></p>