title: Title not showing on R Markdown with knitr when rendering markdown file
body: <p>I am trying to convert a .Rmd file to .md (output: md_document), but the title does not show up on the rendered file. </p>
  
  <p>The title does show up when I try to render the same file as an .html file (output: html_document).</p>
  
  <p>Title shows up on rendered document:</p>
  
  <pre><code>---
  title: "Test"
  output: html_document
  ---
  
  ```{r}
  
  head(cars)
  ```
  </code></pre>
  
  <p><br></p>
  
  <p>Title does not show up on rendered document:</p>
  
  <pre><code>---
  title: "Test"
  output: md_document
  ---
  
  ```{r}
  
  head(cars)
  ```
  </code></pre>
  
  <p><br></p>
  
  <pre><code>rmarkdown::render(my_file)
  </code></pre>
  
  <p>Any ideas why?</p>
  
  <p>I am using RStudio 0.98.1091 and R 3.1.2 on a Mac 10.9.5. </p>
  
  <p><br></p>
  
  <p>The code in between <code>--</code> gets interpreted, as my references are rendered with the following piece of code: </p>
  
  <pre><code>---
  title: "Test"
  output: md_document
  bibliography: ~/mybib.bib
  ---
  
  This is a test where I cite [@post1, @post2]
  </code></pre>
  
  <p><br></p>
  
  <p>The interesting thing is that when I ask for both the html and md files to be generated, the title shows up on the .md file:</p>
  
  <pre><code>---
  title: "Test"
  output:
    html_document:
      keep_md: yes
  ---
  </code></pre>
  
  <p>Shouldn't the output of <code>keep_md: yes</code> be the same as <code>output: md_document</code>?</p>