Gropuing file obtained from instrument window can not be read by LoadDetectorsGroupingFile algorithm

[abuts]

Tried on Wish

 1) Load a workspace (e.g. wish) and show the instrument view (e.g.
 Y-projection)
 2) In instrument view go to pick->pixel selection
 3)Select an area of interest by clicking on a place and dragging the 
    mouse pointer selecting an area.
    Menu appears after releasing the mouse button.
    Choose Create Grouping file->include (or exclude) does not matter
 4) File selection menu appears prompting to pick/name 
    a grouping file.
 5) Save grouping file into desired location. 

When one tries to read this grouping file by LoadDetectorsGroupingFile
the algorithm fails with error, there are no instrument name specified in the grouping file.

If one manually edit the resulting grouping file and sets the instrument name as (tried on Wish):

<detector-grouping instrument="WISH">

The error changes to:

Error in execution of algorithm LoadDetectorsGroupingFile:
SAXParseException: No element found in line 6 column 0. Unable to parse File:

[abuts]

This issue was originally trac ticket 6130