Fix bad completion cache in call(new |)

[RblSb]

Closes vshaxe/vshaxe#668

in this call(new |) case after typing space we trigger display/completion request with completion list, but then display/signatureHelp request overwrites completion cache with this unexpected raise_fields:

Called from DisplayException.raise_fields in file "src/context/display/displayException.ml", line 17, characters 39-53
Called from Typeload.load_type_def in file "src/typing/typeload.ml", line 199, characters 2-110
Called from Typeload.load_instance' in file "src/typing/typeload.ml", line 406, characters 11-43
Called from Typeload.load_instance in file "src/typing/typeload.ml", line 440, characters 10-48
Called from TyperDisplay.handle_signature_display in file "src/typing/typerDisplay.ml", line 313, characters 45-104
Called from TyperDisplay.handle_display in file "src/typing/typerDisplay.ml", line 623, characters 2-37

It seems like in signature mode there should be no raise_fields? Let me know if i'm missing some logic there.