title: Tensorflow 0.12.0rc tf.summary.scalar() error using placeholders
body: <p>Prior to tf 0.12.0rc I've used summary placeholders of the form:</p>
  
  <pre><code>tag_ph = tf.placeholder(tf.string)
  val_ph = tf.placeholder(tf.float)
  sum_op = tf.scalar_summary(tag_ph, val_ph)
  ...
  feed_dict = {tag_ph:[some string], val_ph:[some val]}
  sum_str = sess.run(sum_op, feed_dict)
  writer.add_summary(sum_str)
  </code></pre>
  
  <p>After upgrading to 0.12.0 and changing <code>tf.scalar_summary()</code> to <code>tf.summary.scalar()</code> the use of a placeholder for the <code>name</code> parameter gives the following error:</p>
  
  <pre><code>TypeError: expected string or bytes-like object
  </code></pre>
  
  <p>There is no error if I use a static string for <code>name</code>, but I'd like to change the string as the evaluation progresses. How can I do that?</p>
  
  <p>Minimal example:</p>
  
  <pre><code>tag = 'test'
  val = 1.234
  tag_ph = tf.placeholder(tf.string, [])
  val_ph = tf.placeholder(tf.float32, [])
  scalar_op = tf.summary.scalar(tag_ph, val_ph)
  with tf.Session() as sess:
      writer = tf.summary.FileWriter('/tmp/summary_placeholders', sess.graph)
  feed_dict = {tag_ph:tag, val_ph:val}
  sum_str = sess.run(scalar_op, feed_dict)
  writer.add_summary(sum_str)
  writer.flush()
  </code></pre>
  
  <p>This same code (after reverting tf.summary names) works in TF 0.11.0</p>