title: Can't change params in Rmd documents
body: <p>As explained on <a href="http://rmarkdown.rstudio.com/developer_parameterized_reports.html" rel="noreferrer">this site</a> it is possible to define parameters in the YAML header of a Rmarkdown file and the default values specified there can be overwritten with <code>rmarkdown::render("foo.Rmd", params = list(param1 = "bar")</code>. However when I try this I get the following error:</p>
  
  <p><strong>params object already exists in knit environment so can't be overwritten by render params</strong></p>
  
  <p>Here is a minimal reproducible Rmd document. Let's say the file name is <code>test.Rmd</code>.</p>
  
  <pre><code>---
  title: "Test"
  output: pdf_document
  params:
    name: Andreas
  ---
  
  Hello, my name is `r params$name`.
  </code></pre>
  
  <p>When I now try <code>rmarkdown::render("test.Rmd", params = list(name = "Jordan")</code> it stops with the error written above.</p>
  
  <p>This is my <code>sessionInfo()</code>:</p>
  
  <pre><code>R version 3.2.1 (2015-06-18)
  Platform: x86_64-apple-darwin14.4.0 (64-bit)
  Running under: OS X 10.10.4 (Yosemite)
  
  locale:
  [1] de_DE.UTF-8/de_DE.UTF-8/de_DE.UTF-8/C/de_DE.UTF-8/de_DE.UTF-8
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] knitr_1.11      rmarkdown_0.7.3
  
  loaded via a namespace (and not attached):
   [1] Rcpp_0.12.0      XML_3.98-1.3     digest_0.6.8     MASS_7.3-40     grid_3.2.1      
   [6] plyr_1.8.3       gtable_0.1.2     magrittr_1.5     scales_0.2.5     ggplot2_1.0.1   
  [11] stringi_0.5-5    reshape2_1.4.1   jwiDlst_0.1.1    proto_0.3-10     tools_3.2.1     
  [16] stringr_1.0.0    munsell_0.4.2    yaml_2.1.13      parallel_3.2.1   colorspace_1.2-6
  [21] htmltools_0.2.6 
  </code></pre>
  
  <p>Thanks for you help!</p>