Merge TLS verification patch from Faraday (#340)

Closes #339, CVE-2020-13482

Credit to Mislav Marohnić for original implementation, merged from
Faraday.

lib/em-http.rb

@@ -5,6 +5,7 @@
 
 require 'base64'
 require 'socket'
+require 'openssl'
 
 require 'em-http/core_ext/bytesize'
 require 'em-http/http_connection'

lib/em-http/http_connection.rb

@@ -36,6 +36,62 @@ def connection_completed
     def unbind(reason=nil)
       @parent.unbind(reason)
     end
+
+    # TLS verification support, original implementation by Mislav Marohnić
+    # https://github.com/lostisland/faraday/blob/63cf47c95b573539f047c729bd9ad67560bc83ff/lib/faraday/adapter/em_http_ssl_patch.rb
+    def ssl_verify_peer(cert_string)
+      cert = nil
+      begin
+        cert = OpenSSL::X509::Certificate.new(cert_string)
+      rescue OpenSSL::X509::CertificateError
+        return false
+      end
+
+      @last_seen_cert = cert
+
+      if certificate_store.verify(@last_seen_cert)
+        begin
+          certificate_store.add_cert(@last_seen_cert)
+        rescue OpenSSL::X509::StoreError => e
+          raise e unless e.message == 'cert already in hash table'
+        end
+        true
+      else
+        raise OpenSSL::SSL::SSLError.new(%(unable to verify the server certificate for "#{host}"))
+      end
+    end
+
+    def ssl_handshake_completed
+      unless verify_peer?
+        warn "[WARNING; em-http-request] TLS hostname validation is disabled (use 'tls: {verify_peer: true}'), see" +
+             " CVE-2020-13482 and https://github.com/igrigorik/em-http-request/issues/339 for details"
+        return true
+      end
+
+      unless OpenSSL::SSL.verify_certificate_identity(@last_seen_cert, host)
+        raise OpenSSL::SSL::SSLError.new(%(host "#{host}" does not match the server certificate))
+      else
+        true
+      end
+    end
+
+    def verify_peer?
+      parent.connopts.tls[:verify_peer]
+    end
+
+    def host
+      parent.connopts.host
+    end
+
+    def certificate_store
+      @certificate_store ||= begin
+        store = OpenSSL::X509::Store.new
+        store.set_default_paths
+        ca_file = parent.connopts.tls[:cert_chain_file]
+        store.add_file(ca_file) if ca_file
+        store
+      end
+    end
   end
 
   class HttpConnection