Ensure GUTENBERG_MIRROR env value gets respected

Currently the GUTENBERG_MIRROR environment variable was only getting
respected for CLI usage, not library usage. This change makes the
handling of the environment variable consistent such that the default
mirror can also be set via an environment variable when the project is
used as a library.

gutenberg/acquire/text.py

@@ -17,7 +17,8 @@
 from gutenberg._util.os import remove
 
 _TEXT_CACHE = local_path('text')
-_GUTENBERG_MIRROR = 'http://aleph.gutenberg.org'
+_GUTENBERG_MIRROR = os.environ.get('GUTENBERG_MIRROR',
+                                   'http://aleph.gutenberg.org')
 
 
 def _etextno_to_uri_subdirectory(etextno):
@@ -135,15 +136,13 @@ def _main():
     parser = ArgumentParser(description='Download a Project Gutenberg text')
     parser.add_argument('etextno', type=int)
     parser.add_argument('outfile', type=FileType('w'))
-    parser.add_argument('--mirror', '-m', type=str)
+    parser.add_argument('--mirror', '-m', type=str, default=None)
     parser.add_argument('--prefer-ascii', '-a', type=bool, default=False)
     args = parser.parse_args()
 
-    mirror = args.mirror or os.environ.get('GUTENBERG_MIRROR')
-
     try:
         text = load_etext(args.etextno,
-                          mirror=mirror,
+                          mirror=args.mirror,
                           prefer_ascii=args.prefer_ascii)
         with reopen_encoded(args.outfile, 'w', 'utf8') as outfile:
             outfile.write(text)