fix: remove extra '\n' in netcated HTTP query

Add the '-n' flag to the 'echo' command used to build HTTP request so as
to be more resilient to HTTP server implementation variants.

The current implementation uses the 'echo -e' command to build the query
which has the unfortunate effect to add an axtra '\n'.

Some HTTP server implementations are picky about what you feed them and
might yield unexpected error or corrupted response in this case.

This was discovered through the new implementation of gcp-auth-provider
which uses uvicorn as HTTP server framework.

Fixes: issue #73