Test for not valid letter

[Konstantin8105]

Is your feature request related to a problem? Please describe.
Now, we haven't test for not valid letter.
For example, China hieroglyph or Sweden letter.

Describe the solution you'd like
Add test in file morse_test.go

Describe alternatives you've considered
??

Additional context
Prepared PR #7

[alwindoss]

We should fail the Encode when there are characters that are not recognized by our library. Today it ignores them and translates only those characters that it recognizes