Extract decompression with zlib into GZipDecompressor._decompress_wit…

…h_zlib (#18)

DMARCReporting/decompressor.py

@@ -18,12 +18,15 @@ def decompress(self, gzip_file_path):
             with gzip.open(gzip_file_path, 'rb') as f:
                 return f.read()
         except gzip.BadGzipFile:
-            with open(gzip_file_path, 'rb') as f:
-                do = zlib.decompressobj(wbits=31)
-                data = do.decompress(f.read())
-                remainder = do.unused_data
-                f.seek(f.tell() - len(remainder))
-                return data
+            return self._decompress_with_zlib(gzip_file_path)
+
+    def _decompress_with_zlib(self, gzip_file_path):
+        with open(gzip_file_path, 'rb') as f:
+            do = zlib.decompressobj(wbits=31)
+            data = do.decompress(f.read())
+            remainder = do.unused_data
+            f.seek(f.tell() - len(remainder))
+            return data
 
 
 class ZipDecompressor():