Fixup Base64 node and add better tests

Revert correct <-> behaviour, make encoding info clearer, added type
checking - to close #401 and close #437

parsers/base64/70-base64.html

@@ -4,7 +4,7 @@
         <label for="node-input-action"><i class="fa fa-dot-circle-o"></i> Action</label>
         <select style="width:70%" id="node-input-action">
             <option value="">Convert Buffer <-> Base64</option>
-            <option value="str">Convert String to Base64</option>
+            <option value="str">Encode as Base64</option>
             <option value="b64">Convert Base64 to String</option>
         </select>
     </div>
@@ -22,7 +22,8 @@
     <p>A function that converts the chosen property (default <code>msg.payload</code>) to and from base64 format.</p>
     <p>If the input is a buffer it converts it to a Base64 encoded string.</p>
     <p>If the input is a Base64 string it converts it back to a binary buffer.</p>
-    <p>You can also fix conversion string to base64,and base64 to string if required.</p>
+    <p>You can also fix coding into base64, and base64 to buffer if required.</p>
+    <p>Note: Using "Encode to Base64" will encode an already encoded string.</p>
 </script>
 
 <script type="text/javascript">