Go 1.13beta1 fixes and enablement in tests (#180)

* Fix go1.13beta1 test failures

Fix two legitimate issues, one that broke getUint24 for large values,
another that prevented the example from being tested and displayed:

* travis: add go1.13rc1 to the test matrix

Note: go1.13beta1 fails the build due to a readonly GOROOT, issue 33537.
This was fixed in go1.13rc1.

Author: Lekensteyn

.travis.yml

@@ -19,6 +19,7 @@ os:
 go:
   - 1.11.x
   - 1.12.x
+  - 1.13rc1
 
 env:
   - TEST_SUITE=test-unit

common.go

@@ -1232,8 +1232,8 @@ func signatureFromSignatureScheme(signatureAlgorithm SignatureScheme) uint8 {
 // TODO(kk): Use variable length encoding?
 func getUint24(b []byte) int {
 	n := int(b[2])
-	n += int(b[1] << 8)
-	n += int(b[0] << 16)
+	n += int(b[1]) << 8
+	n += int(b[0]) << 16
 	return n
 }
 

example_test.go

@@ -72,7 +72,7 @@ yuGnBXj8ytqU0CwIPX4WecigUCAkVDNx
 	conn.Close()
 }
 
-func ExampleConfig_keyLogWriter_TLS12() {
+func ExampleConfig_keyLogWriterTLS12() {
 	// Debugging TLS applications by decrypting a network traffic capture.
 
 	// WARNING: Use of KeyLogWriter compromises security and should only be
@@ -116,7 +116,7 @@ func ExampleConfig_keyLogWriter_TLS12() {
 	// CLIENT_RANDOM 0000000000000000000000000000000000000000000000000000000000000000 baca0df460a688e44ce018b025183cc2353ae01f89755ef766eedd3ecc302888ee3b3a22962e45f48c20df15a98c0e80
 }
 
-func ExampleConfig_keyLogWriter_TLS13() {
+func ExampleConfig_keyLogWriterTLS13() {
 	// Debugging TLS applications by decrypting a network traffic capture.
 
 	// WARNING: Use of KeyLogWriter compromises security and should only be