Get keys from json.colHeaders

Fixes an issue where columns of data are placed under the wrong column headers.  Using the column headers (json.colHeaders) as the keys variable ensures that the order of columns is preserved.   

The image below shows the same data.frame printed first from within renderHotable({}) and second from an observe() statement.  Notice that the first column in the original data.frame shows up as the last column, but the order of the headers is the same as the original.
![untitled](https://cloud.githubusercontent.com/assets/7862984/3377486/d4d2dfda-fbdf-11e3-91d4-9dfdd2d3f7aa.png)

I wondered if the data re-ordering issue was a result of using an underscore character, but that did not seem to be the case, as changing it to a zero resulted in the same behavior.  When you have a chance, try it yourself to see if you can replicate my experience.  Maybe it only happens when there are NA values in the data.

Here is an example of how I am using the Handson Table feature of ShinySky: https://dcurrier.shinyapps.io/Plate_Tools/ 
[I tried to set display mode to showcase, but it seems that shinyapps.io does not support that feature yet.]