Refactoring altlink helper (#109)

* Refactoring altlink helper

Also fix a bug not accept a header context-type when
got charset parameter such as
application/meta4+xml; charset=utf-8

* Add test and accept text/plain mime type

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

tests/test_helper.py

@@ -0,0 +1,48 @@
+import pytest
+
+from aqt import helper
+
+
+@pytest.mark.unit
+def test_helper_altlink(monkeypatch):
+
+    class Message:
+        headers = {'content-type': 'text/plain',
+             'length': 300,}
+        text = '''<?xml version="1.0" encoding="UTF-8"?>
+<metalink xmlns="urn:ietf:params:xml:ns:metalink">
+  <generator>MirrorBrain/2.17.0</generator>
+  <origin dynamic="true">http://download.example.io/boo.7z.meta4</origin>
+  <published>2020-03-04T01:11:48Z</published>
+  <publisher>
+    <name>Example Project</name>
+    <url>https://download.example.io</url>
+  </publisher>
+
+  <file name="boo.7z">
+    <size>651</size>
+    <hash type="md5">d49eba3937fb063caa48769e8f28377c</hash>
+    <hash type="sha-1">25d3a33d00c1e5880679a17fd4b8b831134cfa6f</hash>
+    <hash type="sha-256">37e50248cf061109e2cb92105cd2c36a6e271701d6d4a72c4e73c6d82aad790a</hash>
+    <pieces length="262144" type="sha-1">
+      <hash>bec628a149ed24a3a9b83747776ecca5a1fad11c</hash>
+      <hash>98b1dee3f741de51167a9428b0560cd2d1f4d945</hash>
+      <hash>8717a0cb3d14c1958de5981635c9b90b146da165</hash>
+      <hash>78cd2ae3ae37ca7c080a56a2b34eb33ec44a9ef1</hash>
+    </pieces>
+    <url location="cn" priority="1">http://mirrors.geekpie.club/boo.7z</url>
+    <url location="jp" priority="2">http://ftp.jaist.ac.jp/pub/boo.7z</url>
+    <url location="jp" priority="3">http://ftp.yz.yamagata-u.ac.jp/pub/boo.7z</url>
+  </file>
+</metalink>
+'''
+
+    def mock_return(url):
+        return Message()
+
+    monkeypatch.setattr(helper, '_get_meta', mock_return)
+
+    url = 'http://foo.baz/qtproject/boo.7z'
+    alt = 'http://mirrors.geekpie.club/boo.7z'
+    newurl = helper.altlink(url, alt)
+    assert newurl.startswith('http://ftp.jaist.ac.jp/')