Trailing comma with label causes error in parse_params

[jstvz]

Steps to Reproduce:

```{r cars, warning=FALSE,}
summary(cars)
```

will result in:

> options(encoding=''); require(knitr); knit('a.Rmd');
Loading required package: knitr


processing file: a.Rmd
Error in parse_params(params) : 
  all options must be of the form 'tag=value' except the chunk label
Calls: knit ... split_file -> lapply -> FUN -> parse_block -> parse_params
Execution halted

knitr terminated with status 1

NB: Using {r warning=FALSE,} doesn't produce this error.

This was in a largish document so it took a bit to track down. An error report that contains a line number such as in issues #219 and #151 would be helpful for when I neglect to split my documents.

Occurs in RStudio and using Rscript -e "library(knitr); knit('foo.Rmd')".

SessionInfo:

print(sessionInfo(), locale = FALSE)

## R version 2.15.1 (2012-06-22)
## Platform: x86_64-pc-linux-gnu (64-bit)
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] knitr_0.8
## 
## loaded via a namespace (and not attached):
## [1] digest_0.5.2   evaluate_0.4.2 formatR_0.6    plyr_1.7.1    
## [5] stringr_0.6.1  tools_2.15.1

Thanks.

[yihui]

Thanks for the report and sorry for wasting your time on this; knitr will give more detailed error messages in the future so you can see the problematic chunk options.

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