title: $te method doesn't find existing key if it contains spaces
body: <p>I have already made a [issue][1] on the official vue-i18n page, but solving this issue would really help me out thats why I'm asking here too.</p>
  <p>I'm using the translation key directly as the english translation so some translation keys look like this <code>'Next':'Weiter'</code> and others contain spaces or even full sentences: <code>'This text is translated.':'Dieser Text ist übersetzt.'</code>.</p>
  <p>So to prevent a million warnings about the missing english translations I want to check for the key first(<code>this.$te('key')</code>) but this doesn't seem to work when the key contains spaces. It returns false even if the translation key is there and the translation is working.</p>
  <p>Does someone know a solution to this or do I have to wait until someone addresses my issue on the github page/fixes it on the package itself?</p>
  <p><strong>Edit:</strong> The problem is fixed in the latest version of vue-i18n. My other problem still stands though. The missing handler option doesn't prevent the warning being thrown, so this doesn't work either.
  I solved it by adding my own translation method on the vue instance:</p>
  <pre><code>Vue.prototype.$trans = function (key) {
      return i18n.te(key) ? i18n.t(key) : key;
  }
  </code></pre>
  <p>This way it only translates the key if it actually exists, but still will get complicated if I want to use pluralization etc. so its not a long term solution.
  [1]: <a href="https://github.com/kazupon/vue-i18n/issues/1309" rel="nofollow noreferrer">https://github.com/kazupon/vue-i18n/issues/1309</a></p>