Use OpenSSL default ciphers

Twisted default TLS options restricts the ciphers list a bit -- "a secure default"
https://github.com/twisted/twisted/blob/e38cc25a67747899c6984d6ebaa8d3d134799415/src/twisted/internet/_sslverify.py#L1861

We want to be a bit more permissive with Scrapy
(at least as permissive as Scrapy 1.0 was, and which used a default OpenSSL Context)

See https://www.openssl.org/docs/manmaster/apps/ciphers.html#CIPHER_STRINGS

OpenSSL's 'DEFAULT' seems to be reasonable enough.

Fixes #2311

scrapy/core/downloader/contextfactory.py

@@ -13,7 +13,7 @@ try:
     from twisted.web.client import BrowserLikePolicyForHTTPS
     from twisted.web.iweb import IPolicyForHTTPS
 
-    from scrapy.core.downloader.tls import ScrapyClientTLSOptions
+    from scrapy.core.downloader.tls import ScrapyClientTLSOptions, DEFAULT_CIPHERS
 
 
     @implementer(IPolicyForHTTPS)
@@ -46,7 +46,9 @@ try:
             #   not calling super(..., self).__init__
             return CertificateOptions(verify=False,
                         method=getattr(self, 'method',
-                                       getattr(self, '_ssl_method', None)))
+                                       getattr(self, '_ssl_method', None)),
+                        fixBrokenPeers=True,
+                        acceptableCiphers=DEFAULT_CIPHERS)
 
         # kept for old-style HTTP/1.0 downloader context twisted calls,
         # e.g. connectSSL()

scrapy/core/downloader/tls.py

@@ -28,6 +28,7 @@ try:
         SSL_CB_HANDSHAKE_START = 0x10
         SSL_CB_HANDSHAKE_DONE = 0x20
 
+    from twisted.internet.ssl import AcceptableCiphers
     from twisted.internet._sslverify import (ClientTLSOptions,
                                              _maybeSetHostNameIndication,
                                              verifyHostname,
@@ -60,6 +61,8 @@ try:
                         'from host "{}" (exception: {})'.format(
                             self._hostnameASCII, repr(e)))
 
+    DEFAULT_CIPHERS = AcceptableCiphers.fromOpenSSLCipherString('DEFAULT')
+
 except ImportError:
     # ImportError should not matter for older Twisted versions
     # as the above is not used in the fallback ScrapyClientContextFactory