return correct headers for plain text WCPS results

[Dimitar Misev]

A WCPS query like avg($c) returns plain text response, but in the headers there are a few problems:

'File-name': 'NIR.text/xml'
'Content-disposition': 'inline; filename=NIR.text/xml'
'Content-Type': ''

These should be

'File-name': 'NIR.txt'
'Content-disposition': 'inline; filename=NIR.txt'
'Content-Type': 'text/plain'