Don't map text/plain to csv. Refs #4105.

Jquery sets accepts header similar to "text/plain, */*; q=0.01" by
default for xhr requests. Due to this RequestHandler used to set
extension to csv thereby causing View class to look for views under
non-existent csv folders.
Merge pull request #1696 from ADmad/3.0-csv-mapping

3.0 csv mapping