Fix to ignore empty header value.

According to Augmented BNF in the following RFCs

http://tools.ietf.org/html/rfc5234#section-3.6

http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html#sec2.1

        field-value    = *( field-content | LWS )

http://tools.ietf.org/html/rfc7230#section-3.2

        field-value    = *( field-content / obs-fold )

... the HTTP message header `field-value` is allowed to be empty.

netlib/http/http1/read.py

@@ -321,7 +321,7 @@ def _read_headers(rfile):
             try:
                 name, value = line.split(b":", 1)
                 value = value.strip()
-                if not name or not value:
+                if not name:
                     raise ValueError()
                 ret.append([name, value])
             except ValueError: