Syntax highlighting for R code chunks in HTML file

Problem
-------
In the knitted HTML output the R code chunks are not highlighted.

Issue
-----
Calling knit2html() with the options as an empty string changes
the default options. That is
```
knit2html("file.Rmd", options="")   # No syntax highlight
knit2html("file.Rmd")               # Yes to Syntax Highlight
```
Solution
--------
Remove the options argument.

Effect
------
Highlighted R code in knitted HTML file. As desired.