change DTLSv1_2_method to DTLS_method

[lidedongsn]

refer #1380
mContext = SSL_CTX_new(DTLSv1_2_method());
Chrome under M74 （mine 版本 73.0.3683.75（正式版本） （64 位））got “message: Handshake failed, transportName:video, openSSLerror: error:1409210A:SSL routines:ssl3_get_server_hello:wrong ssl version”
ssl/ssl.h
const SSL_METHOD DTLS_method(void); / DTLS 1.0 and 1.2 */
should be ok

[] It needs and includes Unit Tests

Changes in Client or Server public APIs

[] It includes documentation for these changes in /doc.

[jcague]

I don't see that issue with chrome 73.0.3683.86. We actually need this change to keep compatibility with newer versions.

[alokp]

@jcague this PR might be a better solution because per the docs DTLS_method() supports both DTLS v 1.0 and v1.2 but DTLSv1_2_method() will not work with the DTLS v1.0 clients.

[jcague]

@lidedongsn sorry I was wrong with this code. I didn't realize there's a generic method to use both versions. Will try it locally to check it in my local env with newer versions of chrome. Thanks for the contribution!

[jcague]

I confirmed it also works for Chrome v75, thanks a lot!