Bugfix: -o m4a would fail [Fixes #720]

In FFmpeg, a given encoding may not always point to the same format
string.

spotdl/encode/encoders/ffmpeg.py

@@ -76,7 +76,7 @@ class EncoderFFmpeg(EncoderBase):
             + ["-i", input_path] \
             + arguments.split() \
             + self._additional_arguments \
-            + ["-f", target_encoding] \
+            + ["-f", self.target_format_from_encoding(target_encoding)] \
             + [target_file]
 
         return command

spotdl/encode/encoders/tests/test_ffmpeg.py

@@ -48,7 +48,7 @@ class TestEncodingDefaults:
             '-acodec', 'copy',
             '-b:a', '192k',
             '-vn',
-            '-f', 'm4a',
+            '-f', 'mp4',
              target_path
         ]
         return command
@@ -112,7 +112,7 @@ class TestEncodingInDebugMode:
             '-acodec', 'copy',
             '-b:a', '192k',
             '-vn',
-            '-f', 'm4a',
+            '-f', 'mp4',
              target_path
         ]
         return command
@@ -180,7 +180,7 @@ class TestEncodingAndTrimSilence:
             '-b:a', '192k',
             '-vn',
             '-af', 'silenceremove=start_periods=1',
-            '-f', 'm4a',
+            '-f', 'mp4',
             target_path
         ]
         return command