CompoundColumns adds extra columns to dgrid when using IE8 and Ecmascript 5 shims

[gregington]

When using a dgrid with compound columns, a number of extra columns are added to the drid when running with IE8 whilst also using an Ecmascript 5 shim, such as one found at https://github.com/kriskowal/es5-shim.

This is because of a for-in loop that is iterating through the columns array and not using hasOwnProperty. The functions added to the Array prototype are also included in the iteration and result in extra columns.

[gregington]

Added pull request with a fix.

[kfranqueiro]

Thanks for the report. The tricky part of this issue is that columns can be specified either as an array or an object. However, for cases where it is specified as an array, we really shouldn't be using a for-in loop. I've revised the code in two paths I found to be affected by such practices.