Improve Autocompletion of pragmas within instance declarations

[fendor]

Take this for example:

The completion only occurs if it happens between two {-# #-}, and if I select the completion, this is the result:

This is invalid code. Would be better if we did not introduce the second #-}.

In LANGUAGE pragmas at the very beginning of a file, this does not happen, so I presume this requires just a little bit of fine-tuning of the existing code and adding a test-case.

[michaelpj]

Step 1: add a failing test?