Fix issue kurtmckee#318: test failures when cchardet is installed

feedparser imports cchardet or chardet depending on what's installed:
https://github.com/kurtmckee/feedparser/blob/11990ea1d8791acc76c67781f1d2011daf0c3a99/feedparser/encodings.py#L37-L40

Although these libraries are mostly equivalent, they return slightly different
encoding strings, even though both are correct and lead to succesful decoding.
This change allows the tests to be run with either library by accepting both
encoding names as correct.

cchardet detects slightly different encodings from chardet,

tests/test_encoding_helpers.py

@@ -125,7 +125,8 @@ def test_prefix_file_wrapper(factory):
     assert f.read() == b"abcdef"
 
     f = feedparser.encodings.PrefixFileWrapper(b"abc", factory(b"def"))
-    assert f.read(2) == b"ab"
+    assert f.read(1) == b"a"
+    assert f.read(1) == b"b"
     assert f.read() == b"cdef"